Kris, you might pop it into the dryer for a cycle, regular heat, and see if that doesn't fluff it back up. If the bag has been used often it might be in need of a washing. I wash my bags in warm water, light fabric cycle, liquid detergent (i use plain unscented laundry detergent). Then dry, regular heat, until completely dry and fluffy. Your bag may have a special washing instructions tag on it, so follow what it says.

If you've never washed a down bag, first put it in it's stuff sack. Get the water started, detergent added, and about half full in the machine. Submerge your stuffed bag and start slowly pulling it out of the sack. This will keep you from having to squish air out of the bag for the next half hour.