When a loco that uses MM protocol (fx loco or Märklin Digital with 36xx/37xx ref. number) won’t run, there are several potential causes.
First loco at bottom of the screen has to be an MM loco
A bug in the MS2 software that was around with version 1.81 (maybe earlier) and is still around with version 2.30: If the loco on the first screen position is a DCC loco (problem sometimes occurs with an mfx loco at the first position) then locos with MM protocol cannot be controlled from the MS2. Loco detection from the MS2 will work, the lights of the loco will flash and loco address will be recognized correctly – but loco won’r run.
Solution: You must have a loco that uses MM protocol at the first screen position on the Master MS2. It’s not enough to make this change while the MS2 is on, so after selecting an MM loco at the first position, pull the plug from the power supply out for a few seconds (I slowly count to 10 while the plug is out).
Other potential causes
With older MM decoders, the wires of the feeder track must not be swapped. More about swapped feeder wires.