It's a combination of things, but the biggest factor is that the entire offense stinks right now. He has had a few goal-line opportunities that he hasn't cashed in on, but he still gets way too few. I don't think he's on the verge of losing those carries, so I wouldn't worry too much about him, but he won't get as many opportunities to score as most #1 RB's because of that poor offense.
14 Teams, each keep 14 players
QB: P. Manning, Schaub
RB: MJD, Westy, C. Johnson, A. Bradshaw, J. Ringer
WR: TO, Driver, J. Gage, Crabtree, J. Morgan, K. Curtis, E. Bennett, M. Sims-Walker
TE: Witten, M. Lewis
K: Gostkowski, J. Brown
D/ST: Tenn, NE