Have you made sure that you have actually seen all the letters in the game after solving the puzzle from Lukas' father? I would again try going back to every letter just to make sure Nancy has seen them.
Also, here's a spoiler just to make absolutely sure you have the order correct:
All notes are on the bottom row. Number them 1-8 and play: 5, 2, 7, 5, 6
And if you are playing as an Amateur sleuth, it would be helpful to know what is still unchecked on your task list.