I disagree with the assessment.. he might not knock guys out of the hole like I think he should, but he is an effective blocker just the same.
What they are probably talking or seeing.. is short yardage situations where he isn't knocking guys backwards. Personally.. I think that is part of the teaching this quasi Zone scheme.. on a normal run play, they are teaching guys to basically isolate the movement of the defender so the back can read it and cut.
In a traditional drive scheme.. the objective is to run off a particular gap and to knock people out of the hole and as far out of the hole as you can. So the blockers and lead back get accustomed to kicking the defender out of a hole.. much like a short yardage situation.
From the sidelines it doesn't seem like that big of a deal.. but the difference in the hole is huge. In a drive scheme.. you keep churning your legs and pushing the defender.. in a zone scheme you want to immobilize him and allow the running lanes (and cut back lanes) develop naturally and let the back pick the holes, here as a blocker you set up a solid base and lock the defender down. I can see how people think the guy isn't getting an effective hard block.. but that isn't the intent.
So the bulk of the repetitions is that you are stoning the defender.. then in short yardage without a ton of reps they want you to change in up and kick, push and create a hole.
The Zone scheme should provide multiple lanes and the back picks the best one.. a drive the line and lead back creates the hole.. it is either there or not.
Hence why the Packers choose the zone.. they want guys that are effective pass blockers with good movement.. generally they are not good drive blockers because they lack the natural bulk.
Anyway.. back to the FB's.. a young developing player like Johnson probably has been hampered by the Zone scheme in those short yardage situations.. he is being asked 90% of the time to do something other than drive them out of the way.. and IMO.. reps are everything in football.
"The oranges are dry; the apples are mealy; and the papayas... I don't know what's going on with the papayas!"