John,
Nexrad images do come in from ADS-B signals. When on the Internet (WiFi or Cellular) you should be a 0-?? Min in the field. If connected to ADS-B, you should see "ADS-B" in that field. ADS-B Metars/TAFs and Nexrad come in continuously and not in full updates...
The first thing I'd do it next time you are seeing ADS-B for the Metars/Tafs, touch Menu -> About -> Connected Devices and look at the line for Nexrad. Look to see if there is a recent time stamp in the Nexrad line. If not, then please use the Set Wireless Device button to enable the ADS-B Logger and then send a bug report with the ADSBraw.bin file. This will allow us to see what it going on with the connection.
It is possible that you received a Nexrad Image from the ADS-B device and then the signal went weak and we reverted back to show from Internet, but this would be odd and a "failure to update nexrad via ADS-B" scenario. Does your tablet have Cellular data connection or is it WiFi only?