When something performs poorly, then as a rule there is something wrong with the roots.A list here may be quite long: winter damage to the crown, roots eaten by something, plants rotted because of a sickness, roots competition from trees/shrubs, chemical poisoning(artificial and 'natural').You can write more yourself. Proximity to bird feeding bord(sunflower seed will inhibit growth of other plants). Centaurea can inhibit growth of neighbors. and so on.
Walnut tree(even after years)

Maybe it is getting too much sun. I have mine planted where it doesn't get any sun and it does really well there. It never did very well when I had it planted where it was getting sun even just morning sun. I moved it several times until I found a spot where it thrives.

I guess I'd wonder if this was something new this year. Have the astilbes thrived in this location in prior years and this year suddenly didn't appear to do as well? I have over a dozen planted in various beds around my little green acre where they get filtered sun/dappled shade. They're happy as long as they get plenty of moisture. Although it's recommended, I've never fertilized mine.

While astilbe like regular moisture, they also really like good aeration. Often the soil compacts around them so they don't get the moisture and aeration at the roots that they need. They also grow out of the soil in time, so a fresh layer of mulch on them each year can really help.

And they don't like heat, but it was a pretty hot season in most of the country.

I'd rework the soil really good, add tons of organic matter, replant and mulch, and possibly move them out of the morning sun into more shade. Also it is tough to keep them perfect if they only moisture they get is from the hose. They really prefer a naturally moist garden to do their best, but not everyone has that in their gardens.

Good luck,

Chris

The one that didn't even flower was planted last summer, 2010. It did not flower at all, hardly even grew this year. The other one was planted this year, had 2 lousy flower spikes more brown-looking than the promised red (I don't know the variety) and that was it. Perhaps the juniper shrub roots are out-competing them for nutrients. If I moved them under a tree, would that help? We have gotten a regular monsoon of rain this year, plus I filled in when it was dry.