Based on my impression as I was watching, it looked like they used Johnson for between the tackles runs, doing zone blocking with one cut and go.
When it was Nelson & Mozee, they had plays for them sweeping or stretching to the outside to try & get them in space (which makes sense as that's more in line with those backs' skill sets), and it didn't look like we were doing nearly as good of a job in blocking those schemes.
Probably didn't help that Cinci was in a soft zone, so their whole secondary could immediately see those slower developing runs & rally to the ball.
So while EJ looked like a very solid back & the guys behind him did not impress, I'm thinking at least part of that apparent dropoff is the opportunities & schemes.