Sometimes the problem is the plate or the wall. When it is in the electrical installation it usually has to do with the either the box sticking out too far because of something impeding it from the back, in the wall, or, the device (switch) sticking out or cockeyed in the box from splices or too much wire. The plate screw can be too long which will cause it to bottom out before the head is fully set and pull the plate away the more you tighten it.

Take the plate off and see if you can use it as a scraper on all sides of the opening without hitting any part of the box or switch yoke. If you hit something you will have found the problem. If you hit nothing the problem is probably the wall or the plate.

While unscrewing the plate, turn each screw a little at a time and see if the problem self corrects.