Peer unreachable

Hi folks,

I have a problem which is bugging me. I have around 20 users using my PBX and generally they are very happy, but some of them are having problems?they cannot receive calls although if they call OUT it works. If I check the peer status on Asterisk for those users it is Unreachable (if qualify=yes). I played with qualify parameter but without any success.
I noticed if I move the same account to another Sip server (i.e. Brekeke) the problem disappears. I am starting to think that I am missing something in the configuration but I don?t know what.

Please advise.

Any help is highly appreciated