Given the problems everyone else has had with theirs, I suggest lack of water and frost damage.
Will it still flower after frost damage? Can they still be moved to another position now? One of mine is in a dry location so think it suffers there.
All of them have suffered badly in our garden this year and we have had frost damage in previous years and they have come back.
However this year has been constant for frosts so we can only hope that they will come back but maybe not flower as well.