Astilbes like moist soil. Maybe yours is drying out too much between waterings? You could try digging down about a foot and line the hole with plastic. Refill with soil and plant the astilbe. The plastic will help retain more moisture for the plant.
Pam

My guess is they are drowning! Clay soil is the culprit, it doesn't drain very well and therefor will drown the plant.

Here is a link that might be useful: Easy Shade Gardening

Also, clay soil chokes plant roots. You can mix the clay with some compost to richen it up some. That is what i did with mine and so far so good. Also, I have my Astilbe in complete shade (not in clay dirt) and it is bushy with leaves right now.
Good luck with whatever you do.

ahaynie, my guess is we are in the same area. Tennessee?
Clay soil is a pain. I have a shady area in front of house where i'm planting the astible bulbs. Did you start yours from already established plants? I've read a good mix for shady gardens is humus, mixed with a little sand. This is good for breaking up clay soils, lots of OM. I need all the info i can get on starting these guys out! Let me know how it works out. We don't have gutters on front of house, so i don't want to put them just anywhere. Don't want them to drown, or dry out. Ferns like water, so if they died too, my guess is it wasn't too much water. Ferns are also picky about soil pH, but i'm guessing it's the clay soil. By the way, ferns like complete shade don't they?

oh, duh. new to garden web. UT stands for Utah, sorry. Your soil would be totally different from mine! Best of luck to you, though.

I think you are drowning it too. Bottom line why replant the same thing, again. I had a sorry looking astilbe just a foot from many that were thriving. I moved it a foot the other way and it now thrives too. Sometimes they just don't like where you put them.

Makes sense, arcy. I 'm planting astilbe from root, or whatever you call it. I'm gonna give it a try. With perennials, you can always move them! Any root based plant will rot in too much water, from what i've read. Also, the soil needs to be warm. Once again, I hate the red clay soil we have!!! It's very alkaline and not kind to plants. Going to add a little sulfur,and sand, as well as OM to mine.