I had this happen twice. Once domestic and once international. It is not a blow off issue. It was extremely uncomfortable and, as jd2000 said, it was bad on my back.

When it happened on an interantional flight from DME (it got worse thru the flight and eventually the seat cushion was all the way off), I informed the attendant who was apologetic but offered no help. In that case I didn't ask for reseating because folks were pretty spread out sleeping in empty seats. It was my choice to stay put (plus I was traveling with +1). I would have had to ask the FA to ask another passenger to sit up and let me have the adjacent seat. I did want to make sure the FA knew so that she could mention it to maintenance. But it was indeed very uncomfortable and the seat did move if you adjusted your position in the least. I was on the floor a couple of times pushing the seat cushion back in as tight as I could.

If there were seats available, the FA should have given the OP a chance to change seats --- she/he should have assisted the OP in doing this. It would have been OPs decision on whether or not an alternate seat would be better or not.