Sometimes revive seems to be blocked if enemies are stood over the person you're trying to revive (which kinda makes sense I guess). I also sometimes see a delay of a second before the revive spin happens (sometimes leading me to press again, cancelling the revive).
Even so, there have been hundreds of occasions where the mapping of a large number of functions to a single button (likely to be a console-only problem this) leads to some annoying moves. As "Press X to revive" comes up, I tap X, to which the game responds "Did you mean to take cover?" as my character leaps for bits of cover metres away (if only he/she could manage that when I actually want to take cover!). Other times, you simply stand like a lemon as the game says "Oh, sorry, I was miles away, did you press X yet?". Inevitably, the time taken trying to explain to the game that your push of the X button is somehow linked to the request "Press X" leads to you also being taken down, so that you're lying on the floor next to the person you were trying to save. I'm all for difficulty, but inability of software to follow simply instructions is a reason for failure that I'm not great at tolerating.