About 6-8 months ago I purchased an Elite Controller, and other than two small design issues*, it has been great, except the rubber grips. Over time they started to grow, so they would not fit in their recess and that was causing them to become flappy on the ends.
I just took the controller in to a Microsoft Store, and they just swapped out the whole controller.
So if your grips are stretching/growing, take it in and get it replaced.
* The two design issues:
1) It needs a ridge on the batter cover so it is harder to accidentally hit the paddles when setting it down.
2) I'm 6'4" and have big hands, and I would like replacement grips that would be thicker to allow for a comfortable grip for those with bigger hands.
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The lb and rb buttons are also easily broken.