My sessions are getting expired even session.gc_maxlifetime is set to 10800

Hi all,
I have configured my session.gc_maxlifetime to 10800 in php.ini. Even it was set to 3 hours it sessions are getting expired(that too only few) . When i check the php.ini file, i found other two variables session.gc_probability and session.gc_divisor which are set to 0. Could you please let me know what should be done for keeping the sessions live until the time we provided.