there is probably blame in the OL for not having more a productive running game. but i think for the long runs, the blame can go around.
the RBs may have chosen the wrong lanes after getting past the 2nd level. i recall a run by one of the RBs (although it may have been Tate) where CW said if he had cut it out instead of in, he had the whole sideline and would have scored on a long run.
The WRs and TEs (Tate, Floyd, Rudolph) are young and probably aren't blocking well for long enough down field. Grimes is actually a very good run blocker; but he's also a senior.
I think as they improve and focus on a consistent running game, which starts with the RBs making the right reads on holes and cutbacks and the OL opening up those holes, the WRs and TEs blocking downfield will also improve, yielding more long runs.
so get to-it offense.