I had trouble with my 8" Jet after I got it on longer boards. I was pretty sure it wasn't a set up issue as I had checked it pretty thoroughly when I assembled it.

I eventually figured out it was a technique issue on my part, something about how I was switching hands in the middle of the cut. I didn't have the same problem with my 6" Jet, probably because I never tried to do boards that long on it. I'm glad it was me and not the jointer; it was a lot easier to fix my technique than the jointer