captcha validator don't work properly

I'm trying to develop a users model the main part is

<?php

class Users extends CActiveRecord
{

	public $repeatPassword;
	public $repeatEmail;
	public $verifyCode;
	public $rememberMe = false;
	private $_identity;

	/**
 	* @return array validation rules for model attributes.
 	*/
	public function rules()
	{
    	// NOTE: you should only define rules for those attributes that
    	// will receive user inputs.
    	return array(
        	array('username, password', 'required'),
        	// Signup scenario
        	array('repeatPassword, repeatEmail', 'required', 'on' => 'signup'),
        	array('repeatPassword', 'compare', 'compareAttribute' => 'password', 'strict' => true, 'on' => 'signup'),
        	array('repeatEmail', 'compare', 'compareAttribute' => 'email', 'strict' => true, 'on' => 'signup'),
        	array('username, email', 'unique', 'on' => 'signup'),
        	array('email, repeatEmail', 'email'),
        	// Signin scenario
        	array('rememberMe', 'boolean', 'on' => 'signin'),
        	array('password', 'authenticate', 'on' => 'signin'),
        	array('verifyCode', 'captcha', 'on' => 'signup, signin', 'allowEmpty' => !CCaptcha::checkRequirements()),
        	// The following rule is used by search().
        	// Please remove those attributes that should not be searched.
        	array('id, username, password, verification_code, email, email_notification, sms_notification, forgot_password_code, forgot_password_request_date, status, role, create_time, last_visit, last_ip, security_key', 'safe', 'on' => 'search'),
    	);
	}

	public function beforeSave()
	{
    	if(parent::beforeSave()){
        	// for example
        	$this->password = md5($this->password . Yii::app()->params['saltString']);
        	return true;
    	}
    	return false;
	}

}

and a users controller

<?php

class UsersController extends Controller
{

	/**
 	* @var string the default layout for the views. Defaults to '//layouts/column2', meaning
 	* using two-column layout. See 'protected/views/layouts/column2.php'.
 	*/
	public $layout = '//layouts/column2';

	/**
 	* Declares class-based actions.
 	*/
	public function actions()
	{
    	return array(
        	'captcha' => array(
            	'class' => 'CCaptchaAction',
            	'backColor' => 0xFFFFFF,
            	'testLimit' => 1,
            	'minLength' => 7,
            	'maxLength' => 7,
        	),
        	// page action renders "static" pages stored under 'protected/views/site/pages'
        	// They can be accessed via: index.php?r=site/page&view=FileName
        	'page' => array(
            	'class' => 'CViewAction',
        	),
    	);
	}

	/**
 	* @return array action filters
 	*/
	public function filters()
	{
    	return array(
        	'accessControl', // perform access control for CRUD operations
    	);
	}

	/**
 	* Specifies the access control rules.
 	* This method is used by the 'accessControl' filter.
 	* @return array access control rules
 	*/
	public function accessRules()
	{
    	return array(
        	array('allow', // allow all users to perform 'index' and 'view' actions
            	'actions' => array('captcha', 'signup', 'signin', 'signout', 'forget', 'resetPassword'),
            	'users' => array('*'),
        	),
        	array('allow', // allow all users to perform 'index' and 'view' actions
            	'actions' => array('changePassword'),
            	'users' => array('@'),
        	),
        	array('allow', // no one can create, update, or delete these:
            	'actions' => array('edit', 'admin', 'delete'),
            	'users' => array('@'),
            	'expression' => '(Yii::app()->user->getState("role")==="Operator") || (Yii::app()->user->getState("role")==="Administrator")',
        	),
        	array('deny', // deny all users
            	'users' => array('*'),
        	),
    	);
	}

	/**
 	* Returns the data model based on the primary key given in the GET variable.
 	* If the data model is not found, an HTTP exception will be raised.
 	* @param integer the ID of the model to be loaded
 	*/
	public function loadModel($id)
	{
    	$model = Users::model()->findByPk((int) $id);
    	if($model === null)
        	throw new CHttpException(404, 'The requested page does not exist.');
    	return $model;
	}

	/**
 	* Performs the AJAX validation.
 	* @param CModel the model to be validated
 	*/
	protected function performAjaxValidation($model)
	{
    	if(isset($_POST['ajax']) && $_POST['ajax'] === 'users-form'){
        	echo CActiveForm::validate($model);
        	Yii::app()->end();
    	}
	}

	/**
 	* Creates a new model.
 	* If creation is successful, the browser will be redirected to the 'view' page.
 	*/
	public function actionSignup()
	{
    	$model = new Users('signup');


    	if(isset($_POST['ajax']) && $_POST['ajax'] === 'users-signup-form'){
        	echo CActiveForm::validate($model);
        	Yii::app()->end();
    	}

    	if(isset($_POST['Users'])){
        	$model->attributes = $_POST['Users'];
        	if($model->validate()){
            	$activationCode = md5(rand(1, 10000));
            	$model->activation_code = $activationCode;
            	$model->create_time = time();
            	$model->status = 'none';
            	$model->role = 'User';

            	if($model->save()){
                	Yii::import('application.vendors.*');
                	require_once('Setak/EmailGenerator.php');
                	$emailBodyArray = array(
                    	'siteName' => Yii::app()->params['name'],
                    	'email' => $model->email,
                    	'adminEmail' => Yii::app()->params['adminEmail'],
                    	'signupValidatorUrl' => $this->createAbsoluteUrl('users/signupValidator', array('vc' => $verificationCode)),
                	);

                	echo $emailBody = EmailGenerator::generat('signup', $emailBodyArray);
                	$headers = 'From: ' . Yii::app()->params['adminEmail'] . '\r\nReply-To: ' . Yii::app()->params['adminEmail'];
                	$headers = 'MIME-Version: 1.0\r\nFrom: ' . Yii::app()->params['adminEmail'] . '\r\nReply-To: ' . Yii::app()->params['adminEmail'] . '\r\nContent-Type: text/html; charset=utf-8';
                	$emailBody = wordwrap($emailBody, 70);
                	$emailBody = str_replace("\n.", "\n..", $emailBody);
                	//mail($model->email, '=?UTF-8?B?'.base64_encode(Yii::t('users','No-Reply Please')).'?=', $emailBody, $headers);
                	Yii::app()->user->setFlash('signup', Yii::t('site', 'We will respond to you as soon as possible.'));
            	}else{
                	Yii::app()->user->setFlash('signup', Yii::t('site', 'Unknown Error: We have faced to some error in storing data to database please contact with Administrator.'));
            	}
        	}
    	}

    	$this->render('signup', array(
        	'model' => $model,
    	));
	}

	/**
 	* Displays the login page
 	*/
	public function actionSignin()
	{
    	$model = new Users('signin');

    	// if it is ajax validation request
    	if(isset($_POST['ajax']) && $_POST['ajax'] === 'users-signin-form'){
        	echo CActiveForm::validate($model);
        	Yii::app()->end();
    	}

    	// collect user input data
    	if(isset($_POST['Users'])){
        	$model->attributes = $_POST['Users'];
        	// validate user input and redirect to the previous page if valid
        	if($model->validate() && $model->signin())
            	$this->redirect(Yii::app()->user->returnUrl);
    	}
    	// display the login form
    	$this->render('signin', array('model' => $model));
	}

	/**
 	* Logout the current user and redirect to returnLogoutUrl.
 	*/
	public function actionSignout($token)
	{
    	if($token !== Yii::app()->getRequest()->getCsrfToken())
        	throw new CHttpException(400, Yii::t('app', 'Invalid request. Please do not repeat this request again.'));
    	Yii::app()->user->logout();
    	$this->redirect(Yii::app()->homeUrl);
	}


}

captcha validator in signin action work fine but it does not pass in signup action.
what's the problem