IME, there won't be bubbling in the 2 liter bottles. I see some foaming at the top of the solution after I shake it, but normally, there aren't bubbles.
What mixture are you using to make your yeast solution? Small changes can make a big difference. I typically use 2 cups sugar and 1/8-/14 tsp yeast for a fresh batch. Add the sugar to the bottle, fill 80-90% of the way to the top with warm water and dissolve the sugar. Activate the yeast by stirring it around in a little warm water for a few minutes and pour it into the bottle. Shake to mix. This generally gets me through a month. Remember to fill the bottle nearly to the top with water. If you leave a lot of air, it's just more volume of CO2
that you need to produce to get the gas to the tank.
Also, the depth at which you place your diffuser makes a difference. I use pieces of bamboo skewers as diffusers at the moment and I originally had them at the bottom of the tank. The CO2
mixtures for my 29g wouldn't last very long before they'd stop bubbling. I raised the diffuser halfway in the tank, thereby reducing the gas pressure needed, and my mixtures last longer.