Garmin GTX-327 altitude not reporting

Before I flew again from my engine changeover, my transponder checked out and was signed off. I noticed right away it no longer turned on automatically after airborne, but until I requested in flight following I was informed altitude mode is not reporting. Don't know what happened. Looking for ideas.

To state the easy thing first: Is the transponder on 'ALT' (not 'ON)?
Latest FAA guidance (not really that new) is to leave the transponder on ALT all the time, including on the ground.
Next would be to track down the altitude source. Most likely there is an RS232 wire going into the 327 from an altitude source (encoder, EFIS). Check the connections, check the source.