I recently soke to a customer who was getting part of his jacket potato uncooked. he had to think to turn the potato during cooking despite the microwave having a turntable.
The reason was the glass tray itself. The heat generated in the potato by microwaving it was being absorbed by the glass tray that it was sat on. This meant of course that a small area of potato was remaining un cooked. The solution was simple. Raise the potato off of the tray. Either by cooking it on a defrost rack or better still by using a microwave potato baker. The potato baker is the best idea. It makes for more even cooking and it stops the potato rolling around the tray.