The fact that you are having problems is not surprising .
The engineers design the mounts and clips to make sure they will not come lose and the customers have a reason to call and complain !
Since they are not the ones that have to service the units , why worry about the fact that the trim is not removable easily
You may end up having to slide the unit out enough to access the clips from the sides and pry the prongs loose .
Would suggest getting new clips and just snapping the old ones , but the trim may not be strong enough to allow that .