Over time the buttons on your Xbox controller will start to stick, due to over usage or spilling something on it. Regardless, an Xbox controller only lasts three or four months before it starts having issues. However, instead of purchasing another one why don’t you just open it up and fix the problem.
Remove the indention cups that keep each button in place. Repair your Xbox controller by cleaning each button with rubbing alcohol. The best way to do this is with a cotton swab.