I think I've seen Mike Munchak win one challenge all season.
He's not hesitated to use challenges though. In fact, he's used them rather frequently, but he hasn't had a lot of luck.
One reason is just that: luck. Another reason is that he hasn't been using his challenges very wisely.
The game against the Colts was a perfect example. When Andrew Luck ran a quarterback sneak, it appeared the Titans defense had kept him from the first-down marker. Every replay seemed to suggest that was true as well.
However, the refs gave Luck the first down, and Munchak challenged. Now, like I said, every replay angle seemed to suggest Luck didn't make it to the first down, but the play collapsed with 14 or 15 players around the ball in a very small area.
To overturn the ruling on the field, there must be irrefutable evidence that the call was wrong. It certainly looked like the call was wrong to me, but with that many people piled on and around the ball, there's never going to be a clear enough angle to get the evidence needed to overturn the play.
The play stood, meaning that on replay, there wasn't enough evidence to overturn it. No one should've been surprised by that result.
When you lose a challenge, you also lose a timeout, and in the second half of a game, those are critical. Munchak has to learn to be wiser with his challenges.