That happened a lot to me with the DIY. I figured out I had a leak somewhere.
At the end, the problem was in the bubble-counter, the cap was not sealing properly. I had to ditch the original cap, and get a rubber stopper, the kind used in chemistry labs.
To figure out where the leak is, mix a little soap in water, and apply anywhere there might be a leak. With your finger plug the end of the hose going into the tank so that no air escapes that way, and give the yeast bottle a good shaking. The soap will start bubbling where there are leaks.
I tought that I had finally sealed everything up, but overnight the water would creep slowly towards the bubble counter, and during the day, when the yeast got warmer and produced more CO2, I would get about 2 bubbles a minute. That meant most of the CO2 went into the room, not the tank. After I got the rubber stopper, I get about a bubble every 5 seconds. (250 ml yeast mixture for a 5 gallon tank).