Zannah took out Sarro in a moment with sorcery when she got the chance. Plus she was still an apprentice at the time. And Bane attacks her as soon as they stop talking. She had no chance to use it. Plus:
"She couldn't wait any longer. She'd wanted to surprise Bane, slowly gather her strength before unleashing it so he wouldn't be able to properly defend against it. But she knew she wouldn't survive another clash of lightsabers."
It is that easy. Whats Kenobi going to do? He has no experience against sorcery and Zannah only needs a moment to do it. He'll be destroyed.
They had their defenses up against a telepathic attack from what they thought was an insane Bouncer, not a telekinetic attack from a seemingly harmless little girl.
Not necessarily. Kyp Durron retrieved the Sun Crusher from the core of a gas giant as a barely trained apprentice.
As a full-fledged Jedi Master he admitted that he would never be able to replicate that feat.