codeigniter session expires frequently

[eluser]nani[/eluser]
I'm new to this forum..

I have problem with the codeigniter1.7.2 session...

these are my session settings in config file..


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

what my problem is even though i'm browsing the site.. user sessions expires unexpectedly irrespective of the time mentioned in the config settings.

i'm using database session to store user data..
how to fix this issue...