Try these solutions:
1. Check that AXN/MXN can play back the file type you are trying to access
WMA, AAC, MP3, Vorbis, FLAC and WAV can currently be played. The AXN/MXN does not support ALAC, WMA Pro, WMA lossless, RAW, AU or other file types not mentioned.
2. Make sure the files are non-DRM
For USB Media AXN/MXN can only play non-DRM files. Files that have DRM controls cannot be played.
3. Check that your device does not require more than 1A if it is powered from the USB socket itself
4. Check your device is formatted in FAT32, ex-FAT or NTFS format
5. Make sure to have good database management
The way you organise your USB media affects the size of internal database AXN/MXN needs to build dynamically as you browse content. It is good practise to create folders for Artists within which you should have a sub folder for each Album which then containing the tracks for that Album. Libraries with thousands of tracks in one folder will progressively slow down the AXN/MXN's media browsing.