I have a Jet 6" jointer, shorter tables, 3 knives and a "bevel" problem. I'm not at home, so I don't have the exact model number.
I bought it used 5 years ago with it being stored for the last 2 years, and it runs like a charm, BUT, when jointing edges, I am square; when jointing a flat piece of 2+ inches, after 2-3 light passes, I begin to see a bevel in the wood. I define a bevel at the left side being non equal in height to the right side, as checked against the non-jointed side opposite.

Here is what I've done.
1) Removed the fence, checked the out-feed table for flat. Not perfect, but within about .010 mm using my Veritas machined level.
2) Raised the in-feed to dead level against out-feed side with my feeler gauge and checked from the fence to the front over 3 or 4 places - dead flat.
3) Checked diagonally, still good.
4) Set up my magnetic dial micrometer on the out-feed side, checked the head; there is a small variation, about .03 mm at various points, but left to right, it may not be significant - but I don't know (hence the post).
5) Cleaned the slots after the knives and keepers were removed, with brass brush and where rusted, 220 sandpaper; cleaned the keepers after removing and cleaning the bolts, and cleaned the knives. Upon inspection, they were sharp having been done and honed shortly before storage, but there is a slight dip in the middle of the knives - maybe .010mm.
6) Reinstalled everything and began the setting. This may be my problem. I've tried it 2 ways; I adjusted the knives against a piece of flat wood, checking each knife as I tightened the bolts, then checked with the dial indicator, thought I was OK, ran some wood through very lightly perhaps 1/32" 4-5 times - got a bevel cut.
7) I triple checked the fence against the tables many times against both tables, alternately putting the square and the holder (Starrett) against each surface, and made sure it was right. Same result.
8) Loosened the bolts, re-adjusted the knives to equal the height of the out-feed table again, but this time using a steel ruler set on the table until hey juuuuust touched over 3 places per knife. Set the cut to ~1/64, trying to see if I could just get the fuzz (sorry officer) off. Added a little depth of cut, - after 3 passes, it's starting to bevel closest to the fence.

So, although I've tried to moderate my pressure on the work piece, I MAY be adding a little more pressure against the fence side. Feasible? I'm asking.
It could be that cumulative errors in the knife setting and a deficiency in the cutter head alignment/bearings is causing the issue.
My next step will be to move the fence 2-3" into the head area and see if the error is repeatable over the width of the cut.

A helical head replacement for this jointer runs about $500.00 with taxes. Am I better off selling this machine and getting a new 8" somewhere with the helical already installed. I'm a real part timer, just trying to have a little fun in a stressful old age.