This is an oddball, but apparently a known problem. Some older 4- and 5000 Series MFDs may not see N2K AIS messages even though the device is recognized. In fact, it may only apply to Garmin's own VHF 300, a combo radio and AIS receiver, which will still get GPS off the network and put DSC messages on it. The symptom is not seeing AIS targets it should. The fix is sending it back to Garmin.
5212's with serial numbers lower than 159003090 are affected, along with 5208's lower than 157001659. Also 4212's lower than #155003882
and 4208s under #153004541.
Note, though, that the 5212 I've been long term testing is in the "affected" group, but has gotten N2K AIS fine from Simrad and Raymarine transponders (aside from lack of Class B static data message).