The WTR-670 / 680 beltpacks operate in full duplex mode, that is they can transmit audio to the base station while simultaneously listening to audio from the base station.  To successfully do this the beltpack must be on the same group and channel for a specific beltpack slot on the paired base station (group and channel the same). 
To verify that a beltpack is no longer transmitting to the base station (no Tx shown on base for that channel) do the following procedure:
Take a known good beltpack and temporarily change the channel on it to the “bad” channel and verify that comms in both directions work as expected.  This will verify that the issue is with the beltpack and not the base station.
Use the same procedure to verify that a beltpack is working properly, verify that it is on the same group / channels via its LCD display.