Rekordbox IMPORT

I'm trying to import my playlists from an external USB that I had exported to. Have had Rekordbox for ages and this is the first time I've really had a problem however, during import, it gets to about 50% and then says it's complete however only half of the tracks have come across. 

All the playlists are there however if I check on the USB there are 400 songs in one playlist however only 105 have copied over. 

Is this due to a corrupt file or something or is there a bigger problem here? 

Like I said, I've had rekordbox for a while and this is the first time it's started doing this