Could the drought be affecting it? They are a very drought-tolerant plant, but only once established. Sounds like you planted it this year.

I did plant it this spring, but along with all the other things in the new rock garden, including the other Helianthemum "Wisley Pink". I also have been watering the whole area very well throughout the season and everything else is fine. That's what's baffling. I gave it a good push into the soil yesterday to be sure to re-establish good root contact and gave it a diluted feeding and another good watering. Today it's been raining a lot too. We'll see how it goes.

Maybe try a transplant solution or rooting stimulator. It's possible the roots hadn't made good contact.

Do you have voles? Mine have been quite active this summer. :(
Almost every plant I have seen starting to look a bit droopy, when I pushed on the soil around it, it was clear they had been tunneling in the root area. So I have done quite a bit of stomping areas of garden and then watering things back in. (I think I need a cat . . . )

I've never seen voles, nor have any other plants been having any problems. I do think it's possible that the soil has settled down between the large boulders, and maybe has washed away from under the plant a bit. This was a huge project and the boulders are big - 3 to 4 feet or so - and they had fill behind them so it very easily could have settled some. There used to be a bank there going from the upper garden grade down to the lawn and house level, but I had that dug back and now two tiers of boulders make the grade change more interesting, as well as giving me lots of areas to plant creeping and trailing plants.