It sounds like the ring halves a bottoming-out against each other and not applying enough pressure against the scope tube. Either the scope tube is small, or the ring opening is big. Check to see if there is any gap between the top and bottom halves of the rings. If they are tight together with no gap, try filing the bottom surface of the top half so they can close tighter. Either that or apply a strip of friction tape inside the top or bottom half (or both) of the rings. I have had to do this more than once.
