We know about that issue, it can happen when you press too quickly the shoot button. Other things may cause this, but this is the main problem we encounter. Try to hold down the shoot button longer.
If that doesn't solve the issue, you can use the Backspace key to reset a bunch of variables in the game. This may cause some other issues though, we mainly used it to debug the game.
If there's any other issue, I may make a patch to solve the problem.