This is what I found works too. I use Ray Rice. When I sub at 75% fatigue, 5-7 fumbles a year. When I waited until 50%, it was almost three times as many. It's likely something with the fatigue script -vs- the carry logic. To have a good run game, in my experience, you need at least two top quality RB's and a third to eat up a few more carries. Keep them rotating all game long.