There may be various reasons why the wallPen does not recognize the print files.
1. The .wallpen extension is missing from the file.
2. The .tif file was copied to the USB stick instead of the .wallpen file.
3. The USB stick was not ejected correctly.
4. The USB stick has the wrong format, it should always be formatted in exFat.
5. The file was created with an outdated version of the wallPen processor, the latest version can be found here -> https://service.wallpen.com/help/en-us/17/38