Watching some of the game clips, the Rams' blocking was picture perfect a lot of the time, the Cowboys' d-line was spread out a lot, so that if the Rams got two good blocks, there was a big hole for the backs to run through. Also some mistakes by VanderEsch and J. Smith, they went to the wrong side of the run and had no shot to get back into position to make the play.
Good news (if the coaches can see it) is that Smith and LVE are pretty green to the NFL, LVE in particular, they can and I feel will get better at recognizing misdirection and other offensive "tricks". Remember this is a very young defense for the most part, if the Cowboys can hold the same people together and maybe add an Earl Thomas or similar type defensive player here and there, give them another off season, camp and pre-season and we could see a much improved run defense...