Water. Astilbe's hate to be dry.

So you think if I water it religiously, it might flower next year? How much drying will put the kibosh on blossoms for a season? I thought I actually did a good job of keeping things moist this year, since I'd transplanted or brought in a lot of new things I was watering frequently, and we also got plenty of rain. But maybe I just need to put in a soaker hose with a timer or find a less fussy plant.

Astilbe likes rich, moist soil. Try mulching with composted manure especially rabbit manure topped with shredded leaves.

I bought 5 gal. pots one year dead looking in August, but after soaking them well and planting them in composted barnyard manures layered right on my existing soil they bloomed the next summer. Then in spring I removed 2 plants and divided one so I had 3 for another location at our camp planted in April that received no summer water once our rains stopped July 4th. I watered in mid-September just once. Rains came again in October & we mulched the bed. I expect them to continue to bloom.

You might want to try dividing it next spring to renew it. There might be a lot of old woody root.