Got it working - new parameter
Bob, John Stark also suggested re-seating it. He suggested doing it more forcefully the you might think is reasonable and to do it many times - but this did not help.
John also suggested calling Garmin AT support, which I did - and they solved the problem very quickly.
Earlier this week I upgraded the GNS 480 software to V3.2 (however, I think I was missing the altitude data before I did the upgrade). There is a new undocumented parameter in V3.2 software.
In Maintenance Mode, select "MISC". The new parm is ADC. The default is IGNORE. I changed it to USE and bingo - the problem was fixed.