Not sure where you are, but I can think of two possibilities. One is where she runs into you in the hallway, but you teach an afternoon class and get a Cassandra scene with no options except afternoon punishment. (Afternoon classes aren't a great idea, but the build-by-vote thing makes doing Claire a big early priority.) Even if you punish Amy, she won't advance, but Cassandra will. You need to get run over in the hallway again, punish her, then spy.
The second one needs some other events to happen first, as it is a Maxine scene. In one, you get Amy playing with her riding crop, in the second, you get Maxine spanking Amy to make the evidence look more realistic. Which are you missing?