When you were tightening the mounting clamps, did you tap around them with a soft faced hammer? This can help everything to move so that the torque is applied evenly.
I don't know if this will help you but it is worth a try. I have found that sometimes apply a thin film of grease will allow parts to tighten smoothly and with even force preventing slippage. Worked on my bicycle handle bars and seat post, even though it seems self defeating...