I have an Astilbe (peach blossom), but it hasn't feathered this year

Astilbe will simply not develop good flower heads if they dry out at any point during their emergent period. Best plant it in the garden, ideally in a damp spot where it will thrive. They don't make the best pot plants because of this water sensitivity plus they are quite greedy root feeders and soon starve themselves out of the pot.

Thank you for your advice I'll put in the garden. Do they like a certain type of feed?

No particular feed. Just a general liquid feed now and again if that is easier but most soils have enough in them anyway. You could apply a slow release fertiliser (never in the planting hole!) around the base of the plant once a year. Vitax Q4 is the best out there. Keep the plant well watered during dry spells if not in a particularly damp spot.