Asterisk and Cisco phones


We are usign an asterisk version 13 . Cisco phones 7565 ( about 60 phones) Freepbx version 14.

call-manager patch is applied.

Phones are registered, but randomly they all show registering and I see registration requests are going to the pbx, but asterisk does not get these registration request , we have to restart asterisk, then phones are registered and it happens over and over all extensions are sip as well as trunk
any idea why this is happening?