Hmm.. that sounds dangerous. The airstone might be blocking the CO2 from going into the tank. If so, your CO2 generator might go under heavy pressure and blow up.
Or maybe the airstone is clogged. You'll have to unclog/clean it. I read on some other forum about cleaning them with some chemicals but I forgot where I saw it. Sorry.. Maybe you could try searching here or asking how to clean your airstone yourself.
air stone isnt a very good way to defuse the co2. what little co2 your making is being lost. look for something that can turn the co2 into a fine mist. i use a internal filter i also have this bell that brakes up the co2. there are many different ways to do it. i started out with just a air stone. thats all i had but quickly moved to something better because it wasnt doing very much good.