Sounds like what just happened to my wife's 6440. I took hers apart and the three little "nub's" on the back of the Cam fit into a plastic gear that is supposed to turn the Cam which gives you the proper stitch. Well, the plastic gear on ours is broken and instead of turning with the shaft it just sits there while the shaft turns (which means the Cam doesn't turn either). Looks like I have to take it into the shop as I cannot find anyplace on the web that has a parts breakout.