check to make sure there aren't dents in the alignment slots and washers. Sometimes that can happen and when you tighten the screws during alignment they slip into that groove. otherwise there are a lot of great tricks for making sure the table calibration is done well. paper slips under the corners to make sure the pressure is even when it's pressed on the PDMS, shims to adjust the known issue, all sorts of stuff. We had our aluminum block planed since it was uneven, and I currently use a small steel block to give it some extra weight when calibrating. Remember not to over-tighten.