In reply to Sl@te Head:
Sorry if this is teaching you to suck eggs, but if you can get your foot onto the hold then flexibility might not be the problem. Are you sure you're getting fully sat on your heel, with all your weight off your trailing leg, before you try to stand up?
I spent some time working a route with a rockover crux with a mate last year, and he was really struggling until we realised he was trying to stand up as soon as he got his foot onto the hold, without properly shifting his weight onto it first.