There are only four bolts holding the seat down. The rear two are covered by small plastic caps. I think the front two are expeosed. If they have tightened these bolts before and they've re-loosened, then maybe the threads on one of the bolts or the holes (start praying for stripped bolts now) are stripped.

I suggest this because I had an exhaust leak problem years ago that was fixed over and over and over again until the tech got sick of fixing it and finally looked at the bolts. Yes - they were stripped, and the problem finally got fixed properly by replacing the other side of the connection.