That can also happen when your outfeed table is slightly above your
blade height.

bc

wrote:
Recently, any time I tried to flatten one side of a board, my jointer
would start the cut OK but would remove less wood on the last 2/3 of
the board. Eventually my board was wedge-shaped with the front edge
narrower than the rear. I knew this is caused by the infeed and
outfeed tables not being parallel. To get a true read on how much they
are off you need to use a perfect straightedge the length of both
tables. A good method of making a straight "test bar" is in Fine
Woodworking issue 142. I checked all the other adjustments, blade
height, gib screws, table height, etc, and they were all OK. I was
going to shim my outfeed but I was puzzled as to how two perfectly flat
tables can go out of parallel. After all, how much wear can occur in
the gib and dovetail areas from the minor amount of table height
adjusting done on a normal basis? Instead, I removed both of the
tables and cleaned the gibs and dovetail areas. I reassembled and
tested. It worked perfectly. Something, most likely sawdust, must
have built up in the top edges of the dovetails to cause the out of
parallel condition. Shimming the table would have also fixed the
problem but all I would have been doing was compensating for dirty
gib/dovetails.

Dave