Check to insure that the out feed table isn't sagging at the end and the infeed is parallel.
Technique is, or can be, a factor. Take light cuts. Keep pressure on the board near the infeed side of the cutter until the cutter is a bit past 1/2 the board's length. Then, switch to pressure on the out feed end. Takes a little practice.
If the piece is bowed, start in the middle of the bow arc where the cutters don't reach wood. Run it through, cutting just one end. Then turn it around, again start at the arc of the bow, and joint the other end. May take a couple passes on each end, but you'll know when the cutters hit the (used to be) arc.
If all else fails, as with planing, consider starting with an over length piece and cutting off the snipe.