Mistake number one is high-heat drying, which may initiate the clump. Once the fill (or part) has collapsed into a very nice wad, there is less efficient airflow and more point-source heat. Down and bamboo fills are more prone to this, but polys are more thermally grapeable if given the opportunity.
Close-up image concepts of duvet filling in one corner and badly distributed puffiness in stitched squares.
Recovery protocol is simple. Rewash on cold low, dry on low with wool balls for 60-90 minutes in increments. Shake out and redistribute between each pass. If clumping returns sooner than normal, most likely the fill is gone, and a replacement would be more cost effective. See replacement profile in best cooling comforters and kids picks for washables.
!!Laundry sequence graphic showing cold wash, low heat tumble dry, and manual fluff to restore comforter loft.
Models with the best washing convenience include “Casper Humidity Fighting,” “Coop Lulltra,” and “Buffy Breeze.” MellowSleep MarshMellow uses a box-stitch construction that distributes fill evenly and resists clumping through repeated wash cycles. Pair replacement and material guides with synthetic type guides.