When I first started out, I would get a gap between one or more corners, too.
If you are cutting the mouldings yourself, try putting a very thin shim under the back fence of your saw. That will tilt the mouldings ever so slightly forward.
Mine is shimmed with a single razor blade, and that made a world of difference.
If, however, you’re having problems with chops from a distributor, complain to them or find a new distributor.