Another problem that just occurred to me: if the bar is slightly undersized you may not be able to clamp it tightly. Make sure that there is a gap between the stem and faceplate at each of the four bolts when the bolts are torqued. If any of the gaps close up you will be clamping the clamp against itself instead of against the bar. If this is the case a small shim, perhaps made from emery cloth for friction, may enable a secure fit.