failure of apx320 to register in central

We have seven (7) APX 320s on a larger network. There are approximately 25 non-sophos access points on the network as well. We currently have seven Sophos switches and three HP switches on the network.

I cannot get the APX access points to register. They record the error message that they have timed out -- exact error message is "AP did not connect to cloud within the timeout"

This reads like a failure in the connection, but I can register the APs successfully if I use the Sophos Firewall management. 

Why will the APXs register in the Firewall but not in Central? How do I fix this and get the APX registered in Central?