Cant shoot at enemies

[mercn]

Ok for some reason whenever I'm flying or shooting using my gun and the crosshair goes over an enemy it just stops shooting. At first I thought it was issues with my mouse so I go in Counter Strike and test it out but it works fine when I hold it down. Then I spend a couple minutes instant action and try to kill some guys. My gun stops shooting whenever the crosshair is on the guy. Instead I have to press the mouse button manually instead of holding it down. I checked and it isn't because the gun is on semi because same thing happens when I'm flying.