I have the same problem with my HP. I've torn it down to get to the entire mechanism and found that near the rear underside - the gear furthest back sitting on a thin, geared, metal rod - was cracked. This makes the gear "skip" instead of spin. I've dabbed it with super glue and it appears to be working, for now, but to tear down to this point is quite a commitment. Good luck! I'm taking pictures of how to tear this down and as soon as I figure out how, I'll write a fix it guide submission.