Incoming calls - TDM04B

I have recently installed AAH and am trying to manage incoming calls using the TDM04B. The call hunting is working correctly on the PSTN lines. For example, if the first line is in use, then the next incoming call will just keep ringing and Asterisk will not pick it up. What do I need to change to get Asterisk to answer this call?

The outgoing calls work fine. Asterisk just selects the next available line.

Any thoughts?