CodeIgniter session problem

[eluser]Unknown[/eluser]
Hi,

I m newbie in codeignitor,i have problem in session, when i refreshing (ctrl+F5) 2-3 times. my session automaticaly logs out.i dont understand why this happen.

This is my config,php
//session part

$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 7200;
$config['sess_expire_on_close'] = TRUE;
$config['sess_encrypt_cookie'] = TRUE;
$config['sess_use_database'] = TRUE;
$config['sess_table_name'] = 'abc_ci_sessions';
$config['sess_match_ip'] = FALSE;
$config['sess_match_useragent'] = TRUE;
$config['sess_time_to_update'] = 300;

plz help me asap......

Thanks in advance.......