You should remove the bass box, to do that you have to remove the back plate and then unscrew the bass box from the inside of it. Sounds like you might have a piece of trash stuck in one of the reeds. You can pull the reeds up with a thin knife and blow in it, it might dislodge. If you decide to send it back to the builder you only have to send the back plate, not the whole accordion. Who built your box and did you buy it new?