websitemembership timeout

Hi:

I have a members-only portal for a small group based upon cmsb abd websitemembership plugin. It is only accessible via https.

In its first trial by members, one has found that she is being logged out periodically when idle for many minutes. If she is sitting at a page for, say, 15 minutes, and then clicks a link, she is logged out and returned to the login page.

init.php has the default settings as below:

ini_set('session.cookie_httponly', true );
ini_set('session.cookie_lifetime', 60*60*24 ); // cookies are removed after this many seconds of inactiity (set to 0 for session only)
ini_set('session.gc_maxlifetime', 60*60*24 );

Any ideas as to why I'm getting the logouts?
Does this relate to use pf https protocol?

Thanks.

Philip