Board Thread:General Discussion/@comment-33484193-20171112020744/@comment-4010415-20171112043839

I really think it's just that he hadn't absorbed anywhere near as much energy as he had from the giant robot, and thus, when he used his Semblance on Yang, it was not as powerful as it was when he used it on the giant robot. It's like if Yang only got a little bit of energy for her Semblance and couldn't do as much damage as she usually does. (The writers have actually compared Adam's Semblance to Yang's, after all.)

And then there's the possibility that Yang's Aura might have been more focused on protecting her torso with less of it protecting her arm, since her torso contains vital organs and thus takes priority over her arm. Plus, her arm is smaller than her torso is, so that could also cause there to be less Aura there.

Don't forget that Adam walked toward Yang after she landed on the floor and only stopped because Blake jumped in the way. I think he was walking over there to finish Yang off.