I'd probably guess that replacing the bumpers would help. iFixit sells replacement bumpers [product|IF461-020|here|new_window=true]. There isn't a repair guide for this, but you can follow [guide|146815|this|new_window=true] disassembly guide up to the part where the bumper is removed, swap in the new bumper, and then follow the video in reverse to reassemble. If this doesn't work, your best bet is most likely just sending it to Microsoft for repair, or even just buying a new controller.