If it's the very first jump, you need to time the shot about at the same time as you jump. Sounds counter-intuitive, I know, it's a weird physics thing. If you're talking about the second one, spoiler ahead, you're supposed to fall into the ditch and find a pistol you can combine with the shotgun to fire faster.
That is a little counter-intuitive :). But yay, I got to continue! The idea is cool, although I usually just combine all my weapons into one :P. I found the second level pretty hard and coudn't get through. The barrel sometimes threw me completely off the level, haha. Fun game though! The SFXs are nice and it has a lot of potential. Good job!