Asterisk phone's not ringing

I have an asterisk 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 ? ( i didn’t had this , but sinse i upgrade my asterisk to i have this problem )

if it is a bug, is it solved in ?

Nobody an idee?
not just a bug?