I had the same problem but apparently resolved it because it's working again right now. Hopefully, it will stay that way. Here's what I did... I took out the battery and removed the 1.5 mm screw holding the volume-down button into place. I replaced that screw with a 1.6 mm screw and gave it a good tightening. After putting the phone back together, I gave the volume-down button a good hard press and held it down and viola, now both buttons work again. I'll update this if it mysteriously stops working again.