I bought a Rhinox 2000 from aquatic magic on ebay and it has been working okay. I don't have a drop checker or bubble counter, but there are several streams of tiny bubbles rising out of it constantly.
Maybe there is a leak? I have a 2 liter juice bottle, and after I opened it and released the pressure, it took about 4 hours to start producing bubbles again.
Limewood or a Kordon Mist-Air Fine Airstone are inexpensive options you can try if it is still not working for you.
Good luck, hope this helps.