How odd, I've just asked a question about helianthemums! They get straggly in later summer and you need to cut them well back at that point. The glossy leaved ones don't get straggly which is why i'm looking for one!

It could be a number of things, a photo would help. Something chewing on the roots, depleted soil, too crowded & time to divide, not enough sun, weather.

Many thanks for your help. (as I live in north east england, its probably lack of sun! )

I think its more likely that you didn't cut them back
after flowering. Try it this year just in case. Sounds as though your soil is the kind they will like.