Not Being Received by GX2100 (or similar) VHF with AIS

Scenario: The vessel has a VHF radio with an AIS receiver built-in. It also has a Vesper Marine AIS transponder but AIS transmissions of the vessel aren't seen on the VHF radio. As a result, it's not certain if the AIS transponder is working.

If the VHF radio has been programmed with the same MMSI as the AIS transponder (and it should be), then the VHF radio won't show the vessel on it's AIS display. This has been confirmed with a Standard Horizon GX2100.

The same is true if there are two Vesper Marine AIS transponders. If both transponders have the same MMSI, then neither will "see" the other.