Wall mounted - cover falls off

[Axel]

I've mounted the Expert 120 main unit on the wall ok but the input/cable blanking plate won't stay in place.

Before I attack it with tape is there a trick to make it more secure?

Yes. Grab the prongs that hold the cover and gently pull them outwards. You should hear/feel a slight click. Slide the cover on.
Mine used to do this until I discovered this trick by accident.
Hope it works!