I decided to change out the factory rear sight on my Shield 9MM. I had a gunsmith install a DXW BIG DOT sight. I have used the small allen wrench to tighten the rear sight lock screw. The problem is, after I shoot maybe two magazines, the sight becomes loose again, and can be moved. So I tighten it again and again and the same thing happens. It come loose! Any solutions other than take it back to another gunsmith?