When exactly can I revive people in BF3?

When I take the Assault class, I'd like to work as a diligent reviver. But in many cases (about 60% subjectively), although I can see the heartbeat symbol over the lying team mate, I cannot revive him: Nothing happens when I apply the paddles.
And a couple of seconds later he indeed taps out.

This has been breaking my heart and haunting my dreams.

So please help me, my fellow doctors: Is there some mechanism I don't know behind this, like a time to precharge or something like that?

Solution 1:

You need to make sure you use the defibrillator on their torso. If it fails, try again, if it fails for a second time, try going prone.

Keep in mind there are certain times where it might bug out and treat your team mate's body like it's in another location, for example, when someone dies on stairs, and their body rolls to the bottom, every now and then, the game still acts like their body is where they died.