cdj 2000 nexus / rekordbox dj

hi all, I'm wondering if any one can help me please, my cdjs both play fine when on own in rekordbox dj, i then play 2 cdjs and after few moments cdj 1 stops (always seems to be cdj 1), it stops as in does not go around when hit cue, or does not follow the computer ie 6+,10+,wide etc or master tempo(basically the functions on rekordbox i believe), it plays and does what I press on rekordbox but dont follow on cdj if makes sense, any ideas on this please, your help would be appreciated my setup is acer aspire 16gb i3 pc rekordbox dj with licence activated 2 x cdj 2000 nexus 1 x djm 850k 2 x krk 8