It sounds like your lats might be holding you back.
I'm not sure what it's called, but you could try just doing reps with bands or on the Gravitron of just retracting your scapula from the deadhang position.
Or, you could mix things up. You could do retract your scapula and hold for 1-3 sec and then do a full pullup or do at least a 1/3-1/2 pullup.
Even while focusing on the bottom position though, I'd still do some full pullup work so you can easily transition back to full pullups once you've fixed your weak point.