I just purchased a Diamondback Outlook 07. I can't seem to get the seat tight enough to stay in position. It keeps sliding all the way down in the back which causes major discomfort. I keep tighening it but as soon as I hit a small bump in the trail the seat moves again. It won't go any tighter without stripping the bolts.
Has anyone else had this problem? What should I do?

Thanks for the help.