Well, I could well be in the minority when it comes to Senderos so I certainly wouldn't regard my opinion as being the final word tbf.
I think he's someone who's made the odd mistake and it's affected him. I think he became conscious of it and we all know what it's like - a player who thinks too much can make more mistakes, and those who play their game naturally are less prone to mistakes. I think he could be a very good centre back if he can get over it.
I think it's the same with Theo Walcott. I think he just tries too hard... he isn't playing his natural game and it's just not happening for him.
I have no real solid basis for either opinion - it's just a theory.