And just because they're equipped with mu, doesn't mean it necessarily works correctly or crews are familiar with it. Various switches and air brake valves on the trailing unit(s) have to be correctly set for it to be in the trail mode.
These probably have an earlier version of the AAR 27 wire mu system. In reality, most systems only use around 20 circuits the others being spares or for unequipped features. If any of those wires, pins or sockets are loose or broken in each engine's main terminal board, mu plug or cable - choo won't work as expected.