CDJ Track not loaded

Hi,

All my music is either mp3 or wavs and all play fine on rekordbox and on my laptop, however for some reason when i export to USB some tracks will not load and come up with 'track not loaded' and then an exclamation mark will come up next to the track on the cdj display. Like I said, all tracks are MP3's and wavs and will play completely fine on my laptop and the rekordbox software. 

My USB's are formatted as FAT-32 and I have backed up and restored the library several times and even uninstalled and reinstalled rekordbox. 

Please help this is EXTREMELY frustrating. 

Thanks,

Ayman