Have you ever encountered an instance where the knife wouldn't perform a takedown even though you're right behind the enemy? It either gets stuck or just slashes?
As it turns out, Only one knife takedown can be happening in the map at one time for it to work. In short, two people (regardless of enemy or teammate) can't do a knife takedown at the same time...Yikes!
Here's the video that shows it:
Battlefield 3: Only One Knife Takedown Can Be Happening in the Map at One Time for It to Work