Aw that is difficult. I know that feeling. I'm surprised Dora isn't out earlier as robos sometimes come out during the day as well. Do you think she's maybe hiding away a bit since Fifi arrived? I've had that before when a second hamster arrives.
With Fifi though you could maybe get her waking up a bit earlier with some cage noise at feeding time? And tempt her into a tube with a bit of cucumber in?