An idea concerning reasons for not working protector:
So far as I know, the touch-screen uses another technology than Ipad & Co. Infrared rays are running just above the surface to track where you touch with fingers. So "touching" (pushing) isn't necessary at all, just to be on the right place.
I guess, the protector sheet ist TOO HIGH (THICK) and is blocking the IR-rays.
Edit: oh, dwanthny has been faster....