Both had major mechanical flaws in their swings, and refused to change. Right now, Uggla is trying to pull everything, AND is standing way off the plate. So he's getting thrown nothing but outside pitches. Every now and then the pitcher leaves one over the plate, and he makes good contact. The rest of the time he's an easy out. McLouth had to be demoted to the minors to finally be open to changing, Uggla looks to be the same. The manager's answer should be to be stubborn in response. "You're benched until your approach at the plate is right".
This is also the main reason why that contract was a huge mistake. Uggla only knows one approach at the plate. When he gets older and his swing slows down, he won't be able to adjust. Even before this year, if you had to pick one current good MLB player that would probably fall the fastest and hardest in the next few years, most experts would have picked Uggla.
My problem with Uggla is that he's the type of hitter that makes his living by punishing mistake pitches. Elite pitchers don't make mistakes to him, so he can't hit them. He's the kind of guy who will help you get to the playoffs, but will dissapear come October. But the Braves have always loved that type of player (Andruw, Glaus, etc.)