That's because you're mashing RT so hard that you end up moving the analog sticks. Slow down, there's no time limit to how quickly you have to mash RT. If you still can't do it right after that, either get a new controller or return your game because you would be literally too stupid to play it. I'm hoping the last one isn't the case, though.
I came here to chew bubblegum and kick ass, and I'm all out of bubblegum.
I've tried pushing the button slow and I've tried mashing it fast, and I'm holding the right and left sticks where they need to be. My controller is fine, it works perfectly on every other game I have. After a few seconds of trying to break the lock he just stops, forcing me to start again.