Foam can warp or lose shape depending on what is causing the deformity. A warp or lopsided prop can be caused by putting two parts together and not properly lining them up. You either end up with too much material or too little at the end. Then you try to jam the extra stuff in and cause a bulge or tug and pull to make ends meet causing the opposite side to pull and warping the shape.
EDIT: thats from experience.
EDIT: thats from experience.