I had the same problem, it could be a fungal problem.. when I sprayed mine it flowered until very late in the season

Mine are doing the same. But they fell off last year too. I do not know why. I can't see any fungus or mildew on the leaves or flowers. can anyone shed any light on this for me too. Thanks

As I don't like using chemical sprays, I'm going to try this method: Pour boiling water over several crushed garlic cloves, leave for a few hours then top up with cold water. shake well then seive and decant into a spray bottle and spray affected plants. It's worth a try!