Active sync problems, new email stops coming in after a while

Hi there, we've a F5 APM in a HA pair "Active/Standby as an active sync solution configured. The mobile device users connect with active sync to a F5 LTM Load balancer. The LTM forwards the https request to a F5 APM where the certificate is offloaded and where the active sync irules are. What happens exactly is that all works fine for a while. But at some stage, most issues occur the next morning, new emails are not coming in on the mobile devices. It needs some "manual refresh" (And mobile users have to discard some error messages) on the devices to let the email flow again. We haven't got any indication this has to do something with network connectivity, continued pings and other tools of troubleshooting all seems fine. Recognizable? We'd like to pinpoint the problem to get rid of the "the network is always down" atmosphere :)