John,
There's a few "levels" of ADS-B towers, and they don't all broadcast the same info. If I recall correctly, ground towers (for observing surface traffic at larger airports) do not broadcast NEXRAD and a few other products at all, low level towers only have regional NEXRAD, mid & high towers have regional and CONUS NEXRAD. iFly doesn't currently process the CONUS NEXRAD from the ADS-B stream, so if you're in SoCal and the nearest rain is 150ish or so miles away, you won't see it via ADS-B (but the internet download WILL grab it).