Peter James replies: It could be technique or the beds are not aligned to each other. The most common cause is not transferring the pressure to the outfeed table but rather keeping the pressure on the infeed table.
George Saridakis replies: This happens on my Inca jointer/planer and I have found that using a straightedge to ensure the infeed table and the outfeed table are coplaner works for me.
Jon Siegel replies: The outfeed table is not set correctly because it should not be at the blade height, but 0.002˝ or 0.003˝ below the blade height. If you lower the outfeed table, your problem will be fixed. If you lower it too much you will get a snipe at the end. The exact amount to set the table below the blade height depends on the rate of feed and the type of wood.
Bob Oswald replies: Most likely it’s improper use of the jointer. As the board passes across the cutter and establishes itself on the outfeed table, you must transfer all downward pressure on the outfeed side. The infeed side and your left hand must only push the board, not press down on it on the infeed table.
Straightening a bent board requires technique. If you cut the concave side, you will successfully nip off each end until it gets flat. If you put the convex side down, most people will end up with a smooth and still rounded edge because they fail to work the outfeed table. The initial step is to carve a flat spot on the center of the board dropping onto the cutter away from the end. Once you have a relatively long flat spot, you can switch to full length. But be mindful of the downward pressure on the outfeed table.
Fred Chellis replies: A jointer only cuts on the bottom side and has no control on the top surface of the board other than by your hand pressure. Because of this, a jointer cannot make opposite faces parallel to each other. To make a board of uniform thickness you must first flatten one side of the board on a jointer then make the opposite face parallel to it by running it through a thickness planer.