A warped stretcher frame is not an indication of inferior quality, but merely the result of changing environmental influences. There are 4 wedges on the back, which you can carefully readjust with a rubber mallet. Hammer the wedges diagonally deeper into the corners, this will increase the tension again. In the case of larger canvas prints, it may happen that the wedges have to be readjusted after shipping. Important: Do not immediately drive the wedge in as far as it will go. This could result in undesirable deformations in the opposite direction.