There are many potential causes of poster wrinkling. Some of the most common include:
1. Glass placed directly on top of the poster.
2. The poster frame is too tight around the edge of the poster.
3. The poster frame is hung on a cold outside wall.
4. The poster frame is above a radiator or other heat source.
5. The poster frame is in a humid environment (kitchen / bathroom).
6. The poster frame is exposed to outside elements.
7. The poster frame is located in a heavily air-conditioned room.