Act 3. Why didn't Mary Warren just faint for the judge ? Why did she claim that she couldn't do it ?

In my opinion, Mary Warren couldn't faint for the judge because she was unable to faint on demand. You have to remember that the girls in the courtroom played off of one another. One girl would start and the others would follow, they acted as a cohesive group. When Mary was put on the spot with all of the attention on her.... she simply couldn't do it.