Board Thread:WARNING ALL USERS False Info\Spoilers/@comment-96.238.1.154-20130804085459/@comment-5028409-20130805134121

I stated a theory earlier about why Talia, Laura and Peter could transform like that. The Hale Pack is an actual family and it almost appears as if they didn't really take in external members.

This would probably mean that wouldn't have to had to kill the Alphas in the family but simply inherited the role when the previous one has passed away. So it keeps their Alpha line purer and more in touch with the wolf side. I suspect this because Laura managed to become an Alpha even after the rest of the family(possibly Talia as well) died in the fire and even though Peter is supposed to be older than her.

Derek might be unable to transform as easily as the others due to it being tainted twice (once when Peter killed Laura and when Derek killed Peter). And again he doesn't have as much knowledge as Peter so he probably doesn't have the skill to transform