Body oils and dust make down clump, and clumped down does not hold air. Washing brings the loft back, as long as you dry it properly.
01Wash
Use a front-loading machine on a cold, gentle cycle with a down wash. Ordinary detergent strips the natural oils from the feathers. Close every zip first.
02Dry
This is the step that matters. Tumble dry on low with two clean tennis balls and expect it to take two to three hours. Stop every thirty minutes and break up any clumps with your fingers.
If it still feels flat, it is still damp. Keep drying.
03Store
Hang the jacket or keep it loose in a cotton bag. Leaving it compressed in a stuff sack for months flattens the down.
If a jacket will not loft after a wash, our Manali workshop can re-loft it and top up the fill.