It's impossible to really give accurate advise without seeing you jump but I'd imagine it's either because you're failing to set up the jump / penultimate correctly or you just simply can't handle the speed your taking off with which in turn screws up the setup.
I'm always afraid to hammer in the "set-up" cues for LJ as I know a couple of guys that have been mentally crippled by it. One guy that comes to mind was a 7.60 jumper, now is worrying about the "set-up" with his coach who has a phd in biomech and he's jumping 6.8-7.1. I know another in a similar situation with similar performance results.
How technical do you really need to get in the setup? I usually just coach to run at top speed then think "step-pop" for your last two steps while making sure the last step is flat.