I built an air cannon just like you said, however when I fire the cannon, it releases air slowly out of the barrel and does not fire. What is wrong?

On some solenoid sprinkler valves there is a manual handle that you can turn. To fix the problem you must open the manual valve all the way open, then try it. If it still does not work then try turning the manual valve all the way in. It should work when in one of those two positions.