Cakephp，Ajax会话错误

Current Setup :

Cakephp version 2.3.4.

Session configurations are :

Configure::write('Session', array(
    'defaults' => 'php',
    'timeout' => '1440', // The session will timeout after 24 hours of inactivity
    'cookieTimeout' => '1440', // The session cookie will live for at most 24 hours, this does not effect session timeouts
    'checkAgent' => false,
    'autoRegenerate' => true, // causes the session expiration time to reset on each page load
));

Problem

When user login then I write userId in session like this $this->Session->write('user_id',123) and for other API call I read user_id from session like $this->Session->read('user_id');

Problem is sometimes ( manytimes ) $this->Session->read('user_id') gives me nothing like session cookies has been expired.

I read all over on google and stackoverflow and not able to figure out.

Any help Thanks Amit Aggarwal