Three possible reasons:-
One - because they are in pots. Good though they look, it may be that since spring they just haven't had the water they require.
Two - the pruning regime was a bit fierce so they've responded by growing well to regain their stature at the expense of flower.
Three - there are too many stems to support a large flower head per stem, so some have missed out. Like having a plant for the show bench, the fewer stems there are the larger the flower heads.

How long have they been in the pots?

Many thanks. 3 years in the pots, I have kept them well watered when I have been here but when on holiday I think they have only been watered sporadically. They do look very bushy and healthy but disappointing. Change of pots could be the answer although I have a friend whose plant has never been repotted and it comes back in full flower every year. I am grateful for your help.