Keith, crowding and possibly removing foliage prematurely would be my guesses too. As the foliage yellows this summer, I would dig and divide them, and store the bulbs dry till fall. Sometimes when I divide crowded clumps that have stopped blooming, blooms will be sparse the first spring after division, but after that they bloom well.
Frilly, that is odd for Carlton, as its one of the toughest and naturalizes easily. Is the site sunny? Carlton is a mid season bloomer here, but I can see the buds, so I think you'd see them too if they were there.
