Defenses don't fear Miller, for one thing. A-Train was constantly facing a defense geared up for the run Monday night. Opposing coaches believe that they can stop Chicago if they stop the run, and so far it's working.
I think a good strategy against this sort of thing would be to line up every play with 4 WRs. That pulls 2 defenders off the line, forcing a "your guys vs. our guys" line matchup. You have to make the defense respect the pass or it's almost impossible to run.
Bad play calling was certainly another issue. In 4-down territory in the red zone, Miller gets picked off on 1st down, when A-Train should have been running the ball. Defenses seem to know exactly what Chicago is going to do; that's a bad thing.