Usually when you see no CO2 production you have a CO2 leak, which could be at the bottle cap, or anywhere beyond that up to the bubble counter. The seal where the tube enters the bottle cap isn't good for much pressure, so a sticking or backwards check valve makes it leak there, or a sintered glass diffuser may take too much pressure to get gas flow through it, causing a back pressure and leak.
I think I may have this problem... I am using 3 two liter bottles with airline tubing pulled though the cap and sealed with silicone. I am getting a few bubbles through my counter (maybe 5 or 6 per min), but not as many as I would expect given 3 fresh bottles. Plus, if I shake the bottles I catch a whiff of fermentation (that putrid/overly sweet scent). Does anyone know a better way to seal 2 liter bottles? Would epoxy be a stronger seal?
And a totally unrelated question...
I have a 29 gallon moderately planted aquarium (same one as above) on which I am trying a DIY CO2 with a glass & ceramic diffuser. However, I use hang on back filters. I've read that the surface turbulence from these filters dissipates the CO2 from a DIY system so rapidly I may be wasting my time. Does anyone have a success story with HOB filters and DIY CO2?