Your clamp may be jacked up. They are made out of stamped steel. If you continue to have issues, you may want to pull the distributor and check for obvious distortion.
Before you put it back it, do yourself a favor and find an Allen head bolt to replace the slot head clamp bolt. You'll be able to apply a lot more force on it. That flat head screw location makes it difficult to put much tightening force on it with a screwdriver.
