I have found with my CarbonDoser that if I lower the CO2 tank pressure too much, I don't get any bubbles through the check valve. To watch for this, disconnect the CO2 line at the Calc Reactor coming off the bottom of the CarbonDoser, and put that tube in a cup of water to see if there are bubbles coming out. I have also found regular cleaning of the check valve is important. For my system, I want to be absolutely certain I am getting bubbles through, so I use 15 lb. of CO2 pressure (which was recommended by Aquarium Plants who sell the CarbonDoser) with 1 bubble/sec to keep a pH of 7.09 and my alkalinity stays around 8.1 with 100ml/min flow--that's just my system. One other thing, I put a Dwyer inline flow meter to measure the effluent rate. Maybe every fourth or fifth day, my effluent rate drops below the 100ml/minute mark. It's an easy way to watch for that.