I'm sure the answer to this question is likely to be both. But, lets try and just pick one of the choices here
Our running game has suffered and has practically been non-exsistent because:
1) We simply lack the personnel(Running Backs, Offensive lineman) to make it work
2) We simply lack the conviction (By implementing multiple formations, Attempts per game etc) to make it work
I'm personally of the opinion our lack of a running game is a product of #2 moreso than option #1. While, I'm not saying we have the best pieces in place to be a top running team, I do however think we have more than enough to be a decent running team. IMO the #1 problem is How/Formation-wise we chose to run.
I'm sure no-one here would argue that most of our run attempts comes out of that GOD_AWFUL shotgun formation with the back lined-up to either side of the QB. The X's and O's members can weigh-in on this observation. But, with my eyes it seems like this is one of THE WORST ways to try and run the ball when you have:
1) A less than Ideal O-Line
2) A less than Ideal RB
That type of run-formation seems to indicate your Olinemen need to be able to block particularly well or there is going to be serious penetration in the field with no one there to pick it up(Fullback). Secondly, from what I can tell it requires a RB that must have good vision and a feel for line-blocking and must able to break tackles and make people miss at times.
A final observation we seem to limit our running lanes if you wish when we run out of that formation. For example 99% of the time if L.Miller lines up on the right of Tannehill he's either going up the middle or to the left and vice-versa if he on the right. Basically, telling the defense which way we're very likely to run and cutting a portion of the field off like a bootleg play I suppose.
This brings me to my final point. Why don't we run out of more formations? I've noticed when we have ran out of I-Formation we easily averaged 4,5,6 yards on the first attempt( See the Bengals Game). I can only imagine if we actually tried to be consistent in the running game what it would do for Play-Action later on.
Lets be honest we Suck at the running the ball more because of us than what the opponent has done to our running game.