From the in-estimable Chad Micheal:

Follow up to this weekends phone call…..

The L3 NXT transponders are DO-181E compliant. Which means they do not accept “All Call” transmission on the ground. The aircraft has to be weight off wheels for”All Call” to work.
If atc at an airport wants to see the aircraft on ground or if a tech wants to check ground transmission with an IFR6000, they must ping the hex code for the aircraft to receive a reply.
In order to do a ground check using the IFR6000 box, you must configure the hex code in the settings block.

So take that!