User blog comment:Zathronas/Analysis by an author: Volume 2 episode 3/@comment-2006510-20140808072655/@comment-10380701-20140809044443

Wait, that may be it.

In the food fight, Ruby didn't have Crescent Rose. Sure, she flips it around like it's a paper stick, but it's clearly sorta heavy especially when it hits the ground, Ruby's just used to carrying it by this point.

However, combine that with Penny being slightly heavier than a normal human due to being a robot, and you have Ruby's semblance failing.