leaks, baking soda
You first need to be certain that you have no leaks, if there is some gas in the line, but not enough to push down into the tank, then the pressure must be escaping. Every connection is a potential leak.
Aquarium silicone is good in compression but not in tension, so don't count on it to hold an external connection. I like Liquid Nails (a clear caulk type glue from the hardware store, caulk dept) and I've used it even on a unit that was in production and actively leaking and had it hold under pressure when applied to the outside of a cap connection.
Then, more sugar is not better, the sugar can get too high and actually retard the yeast, IME. The water vs sugar ratio is important, that 2 cup sugar is typically with nearly 2 liters of water. For a smaller bottle, less than 2 liter, reduce the sugar accordingly.
Last, I add a teaspoon of baking soda. In tests I conducted using similar bottles, the one with the bakins soda was much better, producing more and longer. If you use bread yeast, it does not tolerate the acidic condition that develops in the bottle, the baking soda gives it an alkaline starting point.
Oh, also, water needs to be not at all hot, just about skin temp is fine.
To proof the yeast, add the yeast to a cup of lukewarm water wiht a pinch of sugar. Let the yeast fall then rise again, look for a tiny bit of foam around the rim of the cup. That means it is good and you have given the yeast a chance to wake up and rehydrate before you plunge it into the sugar solution.