Make sure:

• The MP3 player is a USB storage device.
  The most common mass storage devices are flash drives, memory sticks, and jump drives.
• The MP3 player is using USB 2.0 or USB 1.1 connection.
• The MP3 player is of the correct format.
  Supported format are FAT12, FAT16, FAT32 (sector size: 512 - 65,536 bytes)
• The battery level of the MP3 player is adequate.
  Make sure that the player battery is charged or the battery is fresh.

Note:
In some USB flash players (or memory devices), the stored contents are recorded using copyright protection technology. Such protected contents will be unplayable on any other devices.