Even if the bolt is reasonably tight if you have one of these plastic brackets that is made to accommodate several different pan hole positions you may find that the seat can still move about as the bolt moves within the "spaces" in the bracket. The solution is, get the seat in the correct position and tighten the bolts, then buy a small packet of plastic filler from a car repair shop. Open up the hinged top that covers the bolt heads from view (it is normally just "snapped" down into place) mix the plastic filler and using a spatula fill around the bolt head so that when it sets the bolt is held in position. You will need to do this with both bolts.
Hope this helps.
Yehbaby
October 2009