A potential problem is either related to assembly or knocks in transit where the close coupler cam mechanism is not flat.
Basically remove the bogies and check that the H shaped metal insert which holds the coupling in place is absolutely flat. The piece clips into the barrel and holds the coupling mounting – make sure that it is completely clipped in.
For more general pointers on couplers – the starting point with any couplers is always check that they flat. If they are droopy then you can try swapping couplers around or look at the problem referred to above.
One final potential problem is that the coupler pin might be slightly too long – this should not be a problem for most people but if you have any ballast above sleeper level in the track centre then a simple solution is to trim a very small amount off the coupler pin.