I Can't Cut Anyone's Throat

[Mack the Knife]

sorry for the gruesome title but have a quick question. i'm in one of the russion missions and all my guys have knives. i crawl within striking distance of an enemy, click the knife, i see the guy draw the knife, the reticule turns red, i click the enemy and he puts the knife away, draws his weapon, and shoots!! hello? i have to hit S to cancel it but what gives? ONE time (in fact the first time i tried using the knife) the guy threw his knife and killed the bad guy but that was the only time i got it to work. what am i doing wrong? thanks!