What is time travel?

As in Undersolved questions in Doctor Who, we ponder:


 * Why didn't 5 save Adric in Earthshock/Jericho in FLUX?
 * Why couldn't 11 visit Amy and Rory after the events of the Angels take Manhattan?
 * Couldn't 10 return the victims of the weeping angels back to their own times in Blink?

>muh fixed points