Hotspot users - Idle Timeout ?

I have very strange problems …
I have hotspot setup with some profile where i have Idle Timeout set to 20 min.
Problem is that some users are loged out after short time 2-3 minutes ,
and other work fine … 20 minutes if idle if not idle there is no logout .
Both users are in same profile … and somebody work , somebody not ,
does anybody have some idea why is that hapens?
Im suspect on win XP SP2 , but im not sure…

SP2 on XP gives me some troubles as well.
Try puters with a decent OS or some routers instead and see if it works…

HM … These users have Win XP with SP2 , also one have outpost firewall ..
Under win 2000 there is no problems …
It looks that firewals under XP blocking something … i dont know what … and mikrotik “thinks” that host is idle
(also too fast only 2-3 minute - i have setup for 20 minutes?)
… then hotspot logout user.
Does anybody know how mikrotik find that host is idle ?
ping or something other?
This will be usefull to tell users that disable this replay , port or whatewer …
Also why mikrotik so fast logout user? - this is mikrotik related probably …