I had the same problem. To keep my frames from moving, I stuck one of the small clear (not foam!) 3M mounting squares on each of the bottom back corners of the frames. I put the frames back on the wall, straightened them, then pushed against the bottom of the frame lightly so the squares would stick. They're so light that they don't really stick all that hard to the wall, but they are "rubbery" enough to keep the frame from moving. My frames haven't moved a millimeter since!