I agree with some in that the blocking is just not there, and the scheme isn't working as well as it should. One of the problems that I see is that there is very little second-level blocking.
One of the staples in zone-blocking is that a lineman slips off a double-team to pick up a line-backer which allows for bigger runs. This worked in the first few games, and the running game was earning yards. Then we played KC, who coincidentally have been playing against denver's zone-blocking scheme for years.
KC brought their linebackers VERY close to the line of scrimmage, which seemed to defeat our blocking schemes ability to get good blocks at the second level (LBs)...and the running game hasn't looked nearly as good since. It seems to me that other teams are repeating this strategy and finding success with it, and so far we don't have an answer to it.
Hopefully the run game improves throughout the season, if the Texans learn to run the ball this team will be flat out scary.