Ever try to square a side of a board with a plane. It can be a bit more difficult that it looks.
If the board is off to one side, one thing that worked for me was to offset the blade of the plane. (Please forgive the block diagram). Now the plane will take off more on one side than the other. This let me take off the the bulk of the material quickly.
Don't forget to reset your blade to square as you finish off the job.