Re: How to improve your pullup?
It's tough to say what's going on without having a video example. It could be a lot of things. I know for me, sometimes my normal form is compromised the deeper I step out. This especially happens when I fall into a rut of taking "morning shootaround" jumpers (aka not elevating much) instead of full effort ones.
Again, it's tough to say without seeing, but could it be possible you're not deriving enough power and energy from your legs on those deep shots? I'm not a mega leaper on my J's, but I still must bend my knees and explode in order to create an effortless propulsion. Elsewise, I begin to subconsciously compensate on those long range shots by pushing more with my arms, which then once again compromises my form.
One drill I run myself through from time to time, just as a reminder and refresher as to what the roles of my hands are during my shot - I'll take my left hand (my off hand) and completely straighten it, as if I were preparing to karate chop someone. I'll then take a number of jump shots while keeping my left hand like that the entire time. This will still allow your off hand to help balance the ball (using your lefty palm), but with no lefty fingers on the ball anywhere, your right arm has no choice but to keep inward and push through correctly.
Sometimes, the deeper we're shooting from, the more likely that left hand wants to weasel its way in on the launch process somehow, which then allows the right hand to alter its job, which then leads to a lot more calculations and a less consistent result.
So for now, I guess that's it. Check your legs (are you low and square) and check your hands (is your lefty truly just being used as a means of keeping the ball on its launch pad)?