What type of lumber did you use for the framing ? It can be difficult to get framing lumber, 2x4s, 2x6s, 2x8s, etc that are and will remain perfectly true. Even kiln dried lumber can move and twist. White pine boards, 1x4s, 1x6s, 1x8s will be more stable. Better yet, glued up stock, that is the type you will see that is made from shorter pieces of 1x material, will be the most stable. It's available primed at Home Depot and Lowes or any good lumber yard.
If the lumber you have has not twisted, it can be straightened on a table saw, easier than it can on a joiner. Have a look at this video.
