How queer :-/
I wonder if the other clems are taking Dianas moisture ?
Do you have to water your garden in the main season ?
'I' do because mine's quite a dry one .... if i don't water well a lot of 'my' clems suffer, especially Diana (my favourite clem).

Just thought .....
I take it you prune it back, do you ?
If you don't i wonder if 'that' could be the problem ?

Yes I do prune it back in the spring. My soil is quite light and dries out quite quickly, but a bit puzzled as to why it hasn't happened in previous years as it is with the same clematis companions. Thanks for replying.

If you pruned it, when did you do it? Should be done between end of November and mid February at the latest, down to about 8 inches from the ground.

Ah, sorry, we posted at the same time - when you say Spring, when exactly did you do it this year, what month? Because if it was April or May, you may will have removed the flowering stems...

Yes, quite possibly :-/
I do mine in late winter, about February usually, maybe early March.

I thought it was in Feb, but could be wrong. Will certainly do it this year before March. Thanks for the idea.