Would not tighten up to either picatinny rail - Burris AR Tactical Mount 1/2 inch Picatinny Riser

I tried mount this to my SIG M400, but the clamp would not tighten up to the Picatinny rail. no matter what I tried, it would not tighten. I called SIG tech support, nothing wrong with my rifle. I called Burris tech support, they suggested trying another Picatinny rail. Good idea, I thought. I tried my Bushmaster, it would not tighten. I am returning it to Optics Planet (who was great BTW), and will pursue a different solution.

I can't imagine why this riser block doesn't work, but it seems that several others have the same issue with it.