k, another option might be your hole through the frame is a little off where it should be, causing undue stress? Know what I mean? If its too low or high they(your bolts) would be under tension just sitting there. Does that sound right guys?
If its too low, try adding a washer or two between the plate and the frame, to raise it a little bit. If its too high, some grinding of your plate might be in order. Wait for the guys first though, to make sure.
Or it could be something completely different
Just getting some ideas out in case its happened to someone else before.