If there are no bubbles coming out of your airlines from the Co2 bottles, either the setup is leaking or the mix isnt working. After, of course, checking for leaks (which you have done), I would look into the mix. It is entirely possible that the yeast you used was dead for whatever reason. This was the case for me on my first attempt. I used some yeast we had here at home (not knowing how old it was) that was dead. I have a friend who also does DIY Co2 and he has bought atleast 2 batches from the grocery store that were dead.
If you're doing your Co2 like most suggest, dead yeast would be evident before adding it to the bottles. This is how I do mine. Use a 1/4 cup of warm water (100F-110F). Add a pinch of sugar and a 1/4 teaspoon of yeast. Stir it well enough to break up all the globs and then let it sit for 10 minutes in the cup. The yeast should start foaming up in the cup, if not, it's dead. It wont foam alot, but will foam none-the-less. While you're waiting on it, add 75F-85F water to your 2-liter bottles and 2 cups sugar to each. I also add a 1/4 teaspoon of baking soda. After 10 minutes (and if foaming), add the mix to the bottles.
2 things I would suggest. One, is adding a bubble counter. It's an easy way to check if your bottles are producing. It also allows you to count bubbles every couple of days to watch for a bottle going dead as there is no set number of days a mix will last.
The other is adding several more bottles to your setup. With only 2 bottles, you'll be lucky to get to 10 or 15ppm Co2. Most "experts" will tell you to get, and stay, above 30ppm. I started out with a 2 2-liter bottle setup on my 55g and averaged about 13ppm. Mine started pushing bubbles out after about 6 hours. However, after a day or so, my tank started turning green from algae. I jumped up to 6 bottles now and still have some algae, but nothing near to what it was with 2 bottles. My problem now is unbalanced ferts. Still trying to figure that part out.