Join Date: Jun 2010
Location: Jakarta, Indonesia
IME, the air stone is faulty - for some reason, clogged. If it can release air, use a valve on the other tube (one that works fine). Otherwise, you need to replace the air stone.
*** Not sure about US products, but local products sometimes get clogged - even newly used (I used to keep several spares in case - for goldfish). Only relatively high pressured air could get through.