All of the buttons are potentially kind of flakey. I've had a few go bad, and I'm the kind of person who will tear it down and try to fix the button before I bother trying to get a new one.
The buttons' mechanical action is a metal "dome" that snaps up and down and shorts across two round metal contacts underneath. Sometimes when they're assembled, the metal contacts are not perfectly centered underneath the dome, which makes them malfunction. If this happens, you might need to have the button pressed at very specific angles.
If your buttons are doing this and your phone is under warranty, take it in and get it fixed or replaced. If it's out of warranty, you can have it repaired or repair it yourself. The button parts are taped together, so you can peel them apart and realign them if you feel like doing that.