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If your loco is brand new, return it for warranty repair or exchange.
If it is out of warranty, open it up and check to see the plug from the rear coupler is fully seated in the socket on the circuit board. If it is, see if you can unplug it, and plug in the front coupler into that socket. Now check to see if that coupler opens when the rear coupler button is pressed on the remote. If it does, the rear coupler is faulty. If it does not, the remote is faulty.

Larry