We dont know. Take it to the shop and see what they say.

Below middle A on the staff? The only thing I can come up with that will effect that note on downwards is if the body octave vent is opening as soon as you hit the G key. That may happen if the octave tab at or above where the neck tenon is bent or the neck isn't on straight. It's also possible that a piece of felt or cork that forces the body octave vent closed when the G key is not being used has fallen off. just a guess.