I have had the same two clumps of a white one in the same spots for more than 20 years. They bloom faithfully. Pull up a section that is doing badly to see if there are nematodes or grubs or wireworms eating the roots. You can shake the roots over newspaper and see if there are insects or very tiny worms eating away. A reputable nursery often has people who work there because they know about and love perennials.

We were in a historic drought last year and our astilbe still came up. The larger of the 2 is already putting up flower stalks. They didn't really fill in though until quite recently. Perhaps you are jumping the gun? I'd wait a couple more weeks no?

Do voles eat astilbe roots? At least up north here with the amount of snow we had, the voles had a field day since they didn't have to worry about preditors and many of my plants were chewed at or below ground level.

Also, astilbes do like it a bit moister than some other perennials. How bad was your drought last year, and did they get enough water going into the winter?

I'd nix a drought as a cause since my garden is well watered and we never in my memory had a watering restrictions.
Voles never been an issue in a garden and I'm sure that beside astilbes they could find much tastier plants.
Next week I'll pull one of the clumps and try to see waht's going on.
If no-division cause no harms then it should be something else.

The best book I know of about perennials is The Well Tended Perennial Garden by Tracy DiSabato Aust. She says there are 4 key things to remember about Astilbe 1) they are heavy feeders 2) summer drought is their worst enemy 3) winter wetness is their next worst enemy, and 4) they should be divided every 3 years to maintain their vigor.

Mine decline pretty rapidly if they're allowed to dry out too often for too long, or if they aren't divided regularly. They also really do feed heavily. I seldom fertilize the rest of the garden, but I am vigilant about the Astilbe - they get slow release all purpose fertilizer in the spring, again around the 4th of July...never Miracle Grow.

Voles love astilbe. In our covered greenhouses they actually eat astilbe before touching any other plants, including hostas. If I don't put out bait they can eat hundreds of plants over the winter. Astilbe also sometimes grow out of the soil over time, but if you have been adding mulch I doubt they would have been too exposed, so I would lean towards a rodent problem.