Check you're configuration in the backend:
look for: Login Session Lifetime:
This is the time users stay in the now online list. Decrease it or encrease it for effect.
Bye,
Lennox
LENNOX Official CB 3th Party Developer
Working on the CBRC2 Lennoxonline module Official partner and PR-man of
dcsFlashGames
RELEASED:
Normaly these "users online" modules uses a query on the _session table to identify who's online. As long as a user is in there or the query is cached it stays in there.
I had a similar problem with one of the database-query-cache hacks for Mambo, as it cached the query on the sessions table to long.
In general joomla deletes a user-session on the next access to index.php if the max session time has expired or a user logs out.
It may be a good idea to monitor _session on your site what happens if a user logs out.