Don't get heartbroken. This is going to happen to alot of people. Several possibilites
How your OC is using you. ------- your OC may be using you wrong, sending you on dive plays when the OLine can't create holes. Sending you out on pass patterns while other backs get the carries.
A step up in competition -------- Every season your are going to be going to a different league with different teams. Sometimes you are going to get stuck againg slow-build teams with great DC's who can really shut down an opponent.
Not enough kicking ------- how else is your player suppose to kick ass? LOL
You best bet is to talk to your OC, get a build plan and stick with it. A player's best friend is the coordinator. He can make a stud and dud and vice versa.