@gdwin I have tried this with bots, it seems like it's a collision detection and/or reach problem. You cannot literally move close enough from the back to reach the enemy's head with a knife. Either the knife range should increase, or make the collision box smaller from the rear.
actually, I have tested this out again, it's definitely isn't the range issue. It seems like sometimes the knife "Misses". It could be the really bad hit reg, but I tested this in single player so I don't think it is that either. However, It might be that the swiping motion does not cover the whole swipe, but acts more like a single point, kind of like a bullet. But I could be wrong. All I am sure at the moment is that knifing doesn't work sometimes (I tested by freezing the bots with console menu and getting closest to them as possible)