Panel not dimensionally stable

We bought our children for your children's room 2 whiteboards. A Board is now still in use. The second we discarded after about 1/2 year as it has significantly deformed. It had been in delivering a slight distortion, but we as tolerated. Over time, the distortion became stronger through the thin wooden frame. In the end the lower corner about 10 cm away from the wall. The problem did not occur in the other panel. This was worked from the start straight and served its purpose very well.