Q.  I had to tighten the cleat bolts considerably before they would stop slipping. The hard plastic piece that comes in contact with spd cleat was then bent.
A. There are 3 long fins on the cleat plate which match 3 slots in the main body. The fins should fit snugly into the slots. If they don't the entire cleat plate with remain in an elevated position. Tightening the cleat down will then cause the plate to bend. When fitted correctly, the 3 fins go through the main body and contact the sole of the shoe. This prevents the plate from bending/warping.