You don't say where they're growing, in the ground or in pots, and not much information otherwise to go on, really. What's different about the growing conditions of the straggly one? does it get more sun, and therefore dries out quicker, for instance? Windier position?

thanks - I have updated the question now.

thanks

Well they're obviously not getting quite the same conditions if one is straggly - is the straggly one more exposed, say on the end? Could be its drying out more - these plants are woodland growers, liking dappled shade, not full sun, and shelter from wind. Trim over the straggly one and give it a good cut back next year after it has flowered to encourage new growth.

thanks, I certainly will try this.