Open the bottle and smell. If it's yeasty smelling, and maybe a bit of an alcohol scent, then the yeast are working and there is a leak. Also listen carefully to hear tiny tiny bubbles popping (you can usually see them as well if you look closely.) If the water is fairly clear, rather than cloudy, and you can't see, smell, or hear anything happening in the bottle, you either got a bad bit of yeast or possibly cooked them when you added water that was hotter than you thought. I'd put money on a leak, honestly.