A couple of things that may help. Normal silicone airline tube is porous to CO2 so you can lose quite a lot through the line especially if it is quite long. The proper CO2 tubing will prevent that. Another is the mix you are using. Without something to buffer the acidity that builds up in the reactor the yeast will burn out very quickly and produce CO2 for a relatively short time. Buffering it with sodium bicarbonate will help prevent the acid swing and keep the CO2 production going for a lot longer. No idea on the amount of bicarb off the top of my head but worth experimenting with 1/4 tspns increments each refill.