Are you running after the deathsquito? Jumping while trying to hit it?

Every pitch they make is a strike... how could you miss that?

What are you trying to hit it with?

Seriously now... if you are having this kind of issue I would shut the game down and go to
Library - right click on game name for Properties - Local Files - Verify

If you run a modded game you might want to do a clean install by deleting the folder manually after an uninstall.