I planted this sedum earlier in the summer but it's looking a bit sick. Any ideas what could be wrong?
it does indeed look poorly, but I've no idea what the problem is. Sorry.
I think all you can do is give it some tlc. Once the flowers have gone over, cut the flower stems back to save it wasting energy making seeds (which might finish it off.) In the Spring, I would consider moving it to a different part of the garden just in case it doesn't like the soil where it is currently planted. They don't really like a very rich soil so adding farm yard manure is unlikely to help.