Board Thread:Speculation House/@comment-4830106-20141010002549/@comment-30764484-20141010075118

While the "charging up" isn't always needed, it does take focus and concentration. For example when she showed Weiss how she was really fast, they were completely out of combat in a casual situation. It's when she gets stressed and/or panics that she seems unable to use it, because being stressed/panicking clouds your mind and makes it hard to think clearly, hence why she didn't use it with the Deathstalker and then again in this episode. It's not like she physically can't, she just isn't in the right state of mind.