How loud is the room they're singing in? Self-consciousness can cause a singer to hold back in volume, and less wind thru the pipes can make the resulting effort flat. They need to belt it out in order to reach the higher registers, and this requires confidence. Sounds like they need some enthusiastic abandon to their performance: perhaps, as the Ancient Mariner says, the choice of material doesn't inspire such enthusiasm. Or perhaps they are intimidated by their own volume. If the room is 'boomy,' move them outdoors and see if they let go.