Agitator Bolt will not stay tightened on Kenmore Elite Model # 11022942100?

"Kenmore Elite Washer Model #11022942100--
Hi, My washer first started with the top agitator not turning, so I took the fabric softener dispenser off and the cap underneath it, and tightened the screw that was down in there. Shortly thereafter, it was loose again and not turning, so I went through the same steps, only this time to find that the screw head had broken off and there was only a smooth stud of the screw left. (nothing I could screw back in) I called Sears and ordered a agitator cam, which came a little different than the original, but nonetheless the same. I put it in, this time with a bolt. Now, the bolt will not stayed tightened. Everytime I tighten it down, it starts out agitating perfect, then slowly the bolt loosens, until the top agitator doesn't agitate at all. Please help!!"