Hey, well done! This is a very good effort for a first game, especially one made in a relatively short time. You have a very solid basis for a top-down shooter here, and that's a reason to be proud.
Others have already mentioned the dodgy collision detection with the shots. I think the problem is that the bullets are moving too quickly, so they end up skipping "past" the enemy hitboxes. You could make the bullets' hitboxes much larger, make their movement slower, or increase your project's physics FPS. There's other workarounds you can use (e. g. with raycasting) but that's a little more code.