When you want to push messages to Windows Mobile devices you need either a SMS gateway (also requires SIM cards in all devices) OR you need to open port 3005 FROM your server to the mobile devices. This usually doesn't work if devices are on a cellular network (use SMS then).
If you don't have the SMS gateway the server will record the last known client IP for your device and when you do a push it will try to connect to that IP. This is why it might have worked for x days but now your device rebooted or something and you have a new IP address and the push stopped working.
Possible solutions (can combine):
1) Use SIM cards and SMS gateway
2) Open port 3005 from server to devices. Possible have longer leases or fixed IPs for devices on you network
BR
Peter