Another thing to consider (a tip learned from working on the Olympus scopes and micro-min stuff for years), don't fully seat the screws until all screws have been installed. Kind of the same thought behind not fully tightening the lug nuts on your wheels until they are all seated. Sometimes if you seat some first before putting the others in the excess torque on one side can cause the other screws not to fully seat. Try backing off the other screws and see of the top moves slightly to allow for a full seating of the screws.