On my wife's 755, her volume buttons seem to stop working suddenly. Is this a flaw or did we miss some way to lock out the volume buttons?

Basically, on my 755, I can use the volume buttons (h/w ones on the side) to raise and lower volume during a call, or to change ringer volume when not on a call. The only time my buttons don't work is when it is explicitly locked. Hers, however, seem to not react while on a call or thereafter unless she pops out the battery and replaces it. The buttons seem to work until she switches to calendar or some other app, then they stop working........
Ever seen this?
I think it is time to visit the Sprint store......? (phone is about 2 weeks old)