Are they all the same species? I have a red one, maybe Fanal, which hasn't thrived in several places, while other species are quite happy.

But, maybe you should move the shriveling ones nearer to the thriving one.

I don't know the answer to your specific situation, but my different types of astilbe have had drastically different experiences settling in. One thrived right away and is now humungous and strong. A few were moderately happy the first year. The rest limped along for the first few years and just now are starting to thrive. The weakest ones dried out easily even in shade. I kept them well watered even when they looked bad, and now they are well established.

I love astilbe. In my first home where there was black loamy soil and lots of shade, they thrived and multiplied like gangbusters. Here, with some well loved and amended clay soil, they are persnickety at best. They will crisp up on the dead of summer and perk up in the fall with new growth. Making it through the winter is anybody's guess; even with tons of leaf cover in winter.

I have multiple astilbes growing in part sun on the north & east sides of my garden. All are floriferous and healthy-looking. They seem to like my slightly acidic sandy loam. I've never fertilized them since they've never needed help.

Perhaps all yours need is sufficient time to get established. After just being planted a month, I wouldn't expect many perennials to "put on a show" their first year. Mine are lovely this year thanks to plenty of rain but they've been in the ground 7+ years. I'd guess your plants need a few seasons to establish healthy root systems before they begin to perform up to their potential.