There are several things that can affect the CO2
1. Position of the bottle(s). When bottles are placed below the aquarium or have a long length of tubing, the CO2
pressure has to build much higher to over come the water pressure. (Nomad I would recommend moving your bottle to at least level with your aquarium and shortening the length of tubing).
2. Temperature can play a part, especially in the winter or if your Air Conditioning is set extra low. Cooler temperatures slow down CO2
production. Placing the bottles in a water bath with a water heater can help with this issue.
3. Old yeast. Check the date on your yeast. If it's been sitting on the shelf too long it may be expired.
4. Mixing with too warm water. If you use hot water you can kill the yeast which prevents CO2
5. Leaks in the system. Make sure that you are thoroughly checking for leaks with a leak check solution. (A mix of dish soap and water works just fine.) Check all joins, especially at the bottle cap.