Asterisk 1.6.1.6 phone's not ringing

I have an asterisk 1.6.1.6 running.
the basic setup of an incomming call is:

incomming call -> Queue -> extension

this works fine, but sometimes the extensions are not called. Customers stays in queue and asterisk doesn’t even try to ring an extension.

When is do then a reload the extensions starts ringing.

Is this a bug in 1.6.1.6 ? ( i didn’t had this , but sinse i upgrade my asterisk 1.6.1.2 to 1.6.1.6 i have this problem )

if it is a bug, is it solved in 1.6.1.10 ?

Nobody an idee?
not just a bug?