M.I.
Is she otherwise fully potty trained? I know that my son (3.5) would pee, but not poop, on the toilet when he was first trained. He would go in a diaper, or (yuck) in his underpants, but he wouldn't always go on the toilet. It took some time, but now he is doing everything on the toilet. Would it maybe help to have her in a diaper for a little bit in the evening, to get her to go in that, and then once she has become regular again, try doing it on the toilet again?
Just some ideas, I don't know how helpful they will be.