User blog comment:Zathronas/Analysis by an author: Volume 2 episode 11: No brakes/@comment-3146930-20141024021616/@comment-3146930-20141024235046

Because without using it, she wasn't getting any type of opening from Neo and her attacks were being blocked by Neo's insane recoveries or being blocked by her umbrella. If Yang had used her Semblance, that extra boost of power could be enough to possibly break that umbrella or even provide an opening. Also in the past, just getting angry seemed to trigger it (getting mad at Blake, telling everyone to chill in the forest after a series of odd events, Ursa cutting her hair, ect).

All I'm saying is that she was getting her butt kicked and she didn't use her Semblance at all despite all the factors of her usually using her Semblance being there. Just to use an example of my own, it would be like saying Goku shouldn't go Super Saiyan against a stronger opponent because it would waste his ki, except he would have no shot of winning if he didn't go Super Saiyan. At least if Yang had used it, she may have had a shot.