Think I know what the problem is.
I was previously using my laptop to add music to Rekordbox but then I synced with my PC but since I have been putting tracks on to Rekordbox on the PC these are the ones that will only export to USB.
I did notice previously that I couldn't analyze any tracks that added by my laptop when I was on the PC and the same the other way round.
Is that correct? I thought once both were synced that I would be able to analyze and export on both pc and laptop?