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