files - why aren't my WAV and FLAC working

Recently spent ages copying over files from my recordbox onto my USB. Finally got onto the decks and it didn't recognise my FLAC or WAV files.... any reasons as to why? The decks were nexus 800. 

do i need to reload my files? or change them all to a certain file type? I want to start using cdj's but the process is disheartening and makes me want to stick with serato.

Any help or troubleshooting welcomed!

Thanks