On running plays they'll mark you down at the point of inital contact, not where you actually go down. So, you lose a few yards on the spot.
I did rip a big gain after breaking a tackle and they did spot me where I was finally tackled, so that was a good sign.
Edit: Just played a half with OU against USF and noticed it also ocurred on passing plays. I completed a pass past the 1st down marker, was tackled and momentum brought me back behind the line. So, normally, forward progress gives you the 1st down. Well, not in this game. I was spotted where I hit the turf and didn't get the 1st.
Had several running plays where I spotted a good 5-6 yards back from where I ultimately went down.
Looks like I'll be returning this to Gamestop and checking with them to see if I can try out their other copies of the game. Too bad b/c it's a really good game, but this bug makes it unplayable.
Of course, if EA is still replacing the discs, then I'd go that route, but my guess is no since it's been over 2 years since the thing was released.