Hi Flikety!
Where did you bought the controller? You should have warranty on it. Just return it and get a refund or ask for another controller. Also note that you also can return it to Microsoft self (with the receipt) if the shop where you bought is giving you a hard time.
To answer your question: There is probably something wrong with the rubber or the actual button sticker. This can be repaired, but again, as long as there is warranty on it, please return it and get yourself a new one (of refund).
Good luck!!