I know why Saba lost her magic, now.

You know that one episode where after Zuko joins Aang's team he can barely firebend anymore?
Zuko became incapable because his bending style relied on his anger and hatred.
But in this episode, his loyalties, his motivations, they're all different. He no longer had hatred to feed his fires with.

Saba going indie drastically changed her environment behind the scenes.
At the end of the episode, Zuko re-learns firebending by getting accepted by the two last remaining dragons in the world and seeing how firebending isn't just destructive, but also beautiful.
What this means for Saba remains unclear.