Thats easy, actually. You take the bag out of it's storage bag (a loose cotton one, preferably), grab it by the foot or head end, and shake the heck out of it. Make it sort of undulate. That will 1) break up the little clumps of down, and 2) move the loose down around and redistribute it. You should see a noticeable difference.
It could be the bag is due for a washing, as there could be dirt and grease inside the lining that is causing the down to clump. Get some ReviveX Down Cleaner ($8). If you have access to a front-loading washing machine, wash it on the "bulky/bedding" setting, then stick into a dryer and run it at low until the bag is dry. Some people suggest throwing a tennis ball or two in to break up the down clumps, but the action of the drier should take care of it.
If you don't have access to a front-loading washer (do NOT use a top-loaderthe agitator can tear the down tubes inside the bag), then fill a tub with warm water, put in some detergent, and get the bag in there and give it a thorough wet massage. Re-fill the tub to rinse, gently wring the bag, and dry it. Drying will take forever without any spin cycle, but you will get there in time.
And there you go. Clean bag!