I've heard people complain about the cover up button and how it doesn't work. Generally I've had success with it...except in a big part of the game when i can be squeezing for dear life and my back fumbles anyway.
Finally after fumbling on the goal line in a big spot yesterday I ran the replay back to find that my back NEVER COVERED UP! I had hit the button but the replay clearly showed that he didn't do it.
I went into practice mode and discovered the following: Like the interception button, you have to hit the button well before you'd think. With the back it seems that if you dont' cover right away through the hole the tackle animation kicks in and you never get the cover.
In the times I fumbled I realize it was when I'd gotten through the hole, was now sprinting and then tried to cover as I was about to be hit. The tackle animation took over and I never got the cover up. So frustrating as it is...there is a reason that it didn't work.