Ken,
I think that is pretty normal. The traffic will display on the ground but you need to be in the air to pick up the METARS and Radar (unless you are really close to an ADSB ground station). In my area, I think I need to be at at least 1500 AGL before the FIS-B (flight information system broadcast) information is available. The traffic information you are seeing (TIS-B) is probably from aircraft that have ADSB out (you can see N numbers). I do not believe you would pick up aircraft without ADSB out while you are on the ground as that comes from a ground station also. Also remember, not all traffic will show up. There are lots of good summaries on how ADSB works if you do a Google search.
Also, I am not an expert so I might be incorrect. I have noticed occasionally how some traffic will disappear while I am flying only to reappear a few minutes later. I do believe it depends on how your system processes the ADSB information as to what happens on your display. Lots of information coming in and different apps/systems process things differently.
Keith