Is the problem- the jointer flattens a face, you plane the board to thickness,, then the board cups/twists ?
- then, could be a few things:
-wood has high mc.
-Stress in the board (improper drying, or juvenile tree lumber)
-Unevenly removing wood from 1 face of a board. (seldom happens but it is possible on some cuts/slices of tree)
-double check that your initial machining flattened the face (jointer not set up right)
or you are trying to flatten the face of a board with your jointer and not getting good results.
my guess is the top problem.