The DTM30 may not have a DCC signal to its LocoNet cable. Check its green LED is on; if not, look into the LocoNet cabling.

Is it plugged into the front of a UP3/UP5 type panel (The front panels have a different cabling setup and don't send all the signals required)?
Try connecting direct to the command station with a "known good" cable.

Check that the LocoNet cables are OK. We are aware of several instances where a LocoNet cable that passes a test with an LT1 tester is actually high resistance, affecting the messaging. Usually this is the result of a partly but not fully crimped connector.

Is the jumper (JP1) on the board removed? These should be REMOVED for normal operation, and only inserted for programming on a programming track.

Is the board correctly powered? DTM30 boards should not share a transformer winding with the boosters, or with power fed to some other types of device e.g. DS54 or DAC20. This can lead to "ground loops" with short circuits introduced via the return path.

If you are using a "Zephyr" command station, and you have almost no other devices, then the system may need a "crossover" LocoNet cable or a shorting plug to joint together the centre two pins of LocoNet.