Darius is an interesting subject. Had a decent first year after being drafted in the 2nd and then fell off a cliff only to resurrect himself in Indy. He's not elite by any means but he's a solid corner. What happened with the coaching there? I don't expect them to be perfect but they've invested some early round picks at the position and have seen minimal results.
The Patriots have "missed" on a few prospects, like Butler, because their type of coaching doesn't work for all players. Some players need positive reinforcement or other methods. Part of the reason the Patriots maintain a "different" system and look into the backgrounds of prospects is to find out whether the player can withstand the type of coaching that happens in a Belichick meeting room. Some players are better suited to another environment where mistakes aren't constantly reviewed.
http://espn.go.com/boston/nfl/story/_/id/7518409/bill-belichick-everybody-critic-film-sessions
Mike Reiss had a blog post (sorry, can't find it now) that indicated Butler struggled with this type of coaching, didn't respond to being "picked on" during film sessions, and that he ultimately washed out of Foxboro because of "what happened with the coaching" not because of talent.
This tangent started because of ESC's belief that the TECHNIQUE coaching is an issue. That may or may not be true. But when referencing Butler & coaching, is impossible not to bring up the very relevant coaching style issues that occur (infrequently) when the Patriots miss on a player's learning style or a "guess" about whether that player will be able to take their type of coaching. If there's a problem at that level - can the guy succeed in our coaching system? - then whether the coaches can teach him to turn and find the ball* is ... not going to provide many answers.
* FWIW, almost every very-good or great DB evaluator will tell you this - the turn and find the ball - thing is a big deal for almost every prospect and hard to coach. It is the plague of young corners, everywhere. It is a hard skill to learn, harder to teach, and is a very high level skill that few master. Louis Riddick had some comments about learning this skill back on draft weekend, wish I had taped the ESPN discussion segment. Basically, guys who "look good" doing this at the collegiate level are often guilty of cheating in other technique areas and playing sloppily to peek into the backfield.
Basically, this is no where near as simple as "turn around!" and the Patriots issues drafting/developing DBs is probably the result of a TON of factors, including some struggles in coaching a difficult technical skill.