Knifing on PC question

So, I'm pretty good at the game (from what I can tell). But one thing I cannot get right (besides anything that flies), is knifing. Specifically, knifing so that you grab their dogtags. I have pulled this off only about 3 times. the other 99% of the time, it just does a slash. What am I doing wrong? Do I need to have my knife equipped or should I use the "knife" button?