Quoted from Chisox:The lockbar solution (on Rick and Morty at least) is almost always that the game needs to be completely level side to side or the cabinet isn’t square and the glass won’t be even by the lockbar.
I've found it to be the same on my Ultraman. I have crazy uneven slate floors, and even using slides I need to adjust occasionally due to movement. The left to right adjustment is critical for the glass sliding in enough for the lockbar to drop in.