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.
Instructions:
1.Flip your Xbox controller and find the 7 screws that need to be removed. Once they've been taken out with your screwdriver, turn the controller back over. Pull it apart. Make sure you watch to see if any buttons fall out of place. Set them aside.
2.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.
3.Dab the cotton swab and wipe all around each button. In order to repair your Xbox controller thoroughly, you may have to wipe the inside rim of each button. When you're finished, let them air dry.
4.Check to make sure there isn't any other areas inside your Xbox controller that needs cleaned. If you find something, wet the corner of a wash cloth with warm water and wipe it off. Again, make sure you let it dry. Then reassemble. This will repair your Xbox controller.