Are you feeding them? If they are getting too much nitrogen they will make a lot of green and few flowers. Try next year using a high potash instead - something like tomato fertilizer for example.

or don't even feed them. if they 'think' they are doing badly they will produce more flowers as these make seed and then if the plant dies it has successfully bred. hope that makes sense.

Thank you for your your advice, I will definitely change the feed to tomato and maybe feed less often.

Much less often! Border plants don't usually need feeding unless the ground is impoverished. See what happens with a single high potash feed in Spring if you really want to feed.