What are the best conditions for achillea?

Just looked this up - says they like well drained soil in sun, and it looks like they prefer neutral, and in some varieties, alkaline, soil, though that seems to apply to the rockery species rather than the taller ones.

I am surprised that they aren't thriving - they aren't fussy plants. Do they get enough water?

Yes, they 'do' prefer well drained soil.

(mine are half starved and are doing fine!)

In my heavy clay, the 'Terracotta' series grew too well and ended up flopping all over the place!

Poor, well drained soil in full sun is preferred but if you prepare the soil beforehand with some horti grit, should be fine.

Well, from these replies I think they might not have had enough water or sun. I will move them and apply plenty of grit! Thank you all very much.