the problem with this is that it is hard to know who is to blame here. Is he just not that good and just hasn't developed because he's not very good or are the coaches not very good at developing him?

As the father of a college football player I can tell you this, coaches do not really have the time to develop these guys individually. The guys that really improve do most of it on their own not with the coaches. The amount of time the coaches are allowed to spend with these guys by NCAA rules is pretty limited and for most times of the year they aren't allowed to be anywhere near them and a practice field. Individual skill work is almost completely on the player, coaches have little time for that.

My son in a QB, I always assumed that the QB coach spent a bunch of time with the QB's working on their throwing, footwork, form, etc but that is not the case, they don't have the time to do that. Most of their time is spent on team and scheme specific stuff and very, very little on individual skills development.

So if Devin isn't getting any better the majority of that is on him, not the coaches.