Achillea - lower leaves droopy & yellow
I'm a newbie gardener with a southwest facing roof terrace. I bought an achillea pomegranate 3 weeks ago and repotted it into a larger pot, perhaps unwisely using potting compost. (I've since discovered they prefer less rich soil... Oh dear.) I watered it well after repotting, and for the first week, but then the lower leaves started to droop and turn yellow, so I wondered if I'd been over-watering, especially as the label says 'drought-tolerant'. For the last 2 weeks I've watered very sparingly (it's been hot and dry), but the lower leaves are still drooping and yellow. The soil now feels v dry. At the same time, the top of the plant looks healthy and like it's about to burst into blossom. Am I underwatering or overwatering it, or is it just unhappy in rich soil? Any advice on how to make it happier would be really welcome!
Many thanks in advance.