Do you have deers in your neighborhood? I had similar problems with my newly planted hibiscus last year. They came in and munched away the leaves and buds. This year they ate up the agapanthi buds - they sheared them just like you had described - as if some vandal had used scissors to cut them off neatly. I would not have believed that this was deer-work had I not caught them red-handed one night. Try placing deer-block nets around the hibiscus. If deers or other animals were eating them, you won't miss your flowers any more. I hope you donot have vandals in your neighborhood.

Not sure but sounds like a twig Girdler