1. Verify file is a .MP3 file
2. Verify that the file does not exceed 128Kbps bit-rate.
3. Verify that the file does not exceed 48KHz sample-rate.
4. Verify file is not encrypted. Encrypted Messager USB units will only play Sonocrypt (.sc3) files.
5. Verify the USB flash drive is formatted either with FAT or FAT32. The Messager USB does not support NTFS file system.
6. Some USB flash drives have a lock on them - verify the drive is not locked.
7. The file that is to be played should be in the root on the USB drive, not in a folder. (Though files can be stored in folders on the USB drive, as the unit will ignore any folders.)
8. Try another file in the USB player, If you don't have the sample file that came with the USB you can find it attached to this article.