User blog comment:ObiWanCokeNosey/You know what really grinds my gears?/@comment-4010415-20150420153047/@comment-1913284-20150426204501

She was outrunning the Death Stalker without her Semblance, so assuming she didn't know about the Nevermore's feather-spear thing she just had no reason to use it then. That she didn't use it after her cape was pinned suggests she couldn't; it might not be strong enough to tear her cape free, at least not when accelerating from a complete stop.

Similarly, when running from Roman she had gained a decent head start with her Semblance, and certainly didn't know his cane had a grappling hook.

We also don't really know enough about Aura and Semblances yet. Using a Semblance likely has some Aura cost; maybe Ruby's is high enough that using it for long dashes would compromise her Aura shield, making it actually a bad idea when things are trying to kill her and/or she's in hostile territory where the current threat is likely not the last. Alternatively. activating a Semblance could just require more concentration than a panicking fifteen-year-old can muster.