I would move the bully and one of the other girls together to see if that works out first. Sometimes removing the bully will disrupt the order of things enough to let the rest of the mice settle into a new hierarchy. There could be several reasons that mice can suddenly start to display more aggressive behavior such as being able to smell a male, new toys, the bullied mouse is ill/weak, new cage mates, not enough sought after food or living space, or just plain old bad manners, lol.
Hope things settle down for you!
Hope things settle down for you!