I Understand your situation i have the same problem sometimes if i have something else open in the backround like a music player.
When i fire example : Spraying with M4A1 on a medium distance , crosshair in perfect position on the enemy yet they take no damage , as if you shoot right through them.
I Have had this problem a few times , i'm unaware of a fix other then running less programs upon playing SF , perhaps change your resolution settings "in-game" not your Monitors settings.
Use the options menu in SF.
Customize your in-game settings to see if the issue goes away & if it does stick to those settings.
Not sure if it will help you but it did for me a few times.