If you watch the good running 2nd rowers like Graham,Lewis,Frizell,Gillett,Cordner and Cooper (to name a few) they position themselves deeper.That gives them time to use their prop as a decoy on opposing 2nd row, catch the ball on the outside, so as to run at the half or 5/8.JJ stands too flat up in the line, thus making him a target to get held up by 2nd row whilst the prop smashes him.His options become limited to quickly passing out wide or try to bust through the 2nd row before prop and/or half/5/8 tackle him.From lock he can stand deeper,he will be able to inject himself on the outside of the decoys (props) on both sides of the ruck.This increases his ability of finding a gap in the middle,have more time to put the outside backs through a gap and execute cut out passes or shift the ball back to the inside to his halves or fullback.