Probably the weather - too cold/too wet/too hot/too dry.

Could also be getting overcrowded - how long has the plant been in place and how big is it?

Repeat flowering in some varieties is encouraged by pulling out the older flower stems. We had a clump of these in our garden when I was a kid (50+ years ago) and my mother would pull a vase full of fresh flowers from it every week for the whole summer.

Well thanks for that, the plant has been in the same position for two years now. It may be to wet but I must say it looks very healthy.
It is about 2ft in circumference.
I have pulled all the finished flowering stems it seems to be after that it stopped producing more buds.
Such a shame because I just love the flowers.

I think pulling them before the flowers are spent is the way to go, Pancy. Try taking 1/3 of the flowers when they just start to open - they'll last for ages in a vase indoors. And keep replenishing with 1/3 - 1/2 depending on how many new flower shoots it puts out.