I had a case kind of like this, after the day vfr x-country, and the student doing very well, he started to slack off thinking he knew it all. His night x-country planning came back sloppy and his checkpoints weren't very good, and he wouldn't choose new ones. So in the air, I failed everything on a new moon night, he had no navigation except his compass and map. Well he learned pretty quickly that it's very easy to miss checkpoints, not know where you are, and overfly your airport by 20 miles before being told it's behind him.
It sounds mean that you have to really overload them and stress them out, but they understand afterwords, and they will rock the next fligiht.