Hmmm...I think simply letting it dry out really well might solve the problem.
Down that is old and abused is not a detriment. The down cluster will sometimes break a barbule off. This is GOOD. Rather than being compressed by the core, it is now free to loft. Soo, you can pick up a bit of better performance with old down.
The protiens, similar to your finger nails, have the oils embedded in them. Unless you used a detergent instead of a soap, it is almost impossible to wash these out. I generally wash my bags after a couple weeks of use.
Generally, soap mixes with oils and water helping to remove dirt. It does not break the bonds where it is embedded in the down feather. Detergent will encapusulate oils, pulling them out...along with the dirt. Also, ruining the down feather. Sort'a like washing your hands in acetone. It will dry the skin. As far as I know, there is no way to replace the oils that have been removed by a chemical process(detergent.)
Even the detergent left in comercial washers is bad. You should scrub down everthing visable before washing your bag. Over time, this stuff will destroy downs effectiveness.
Once the lanolin-like oils are gone, the feathers get dry, brittle and break easily. They tend to clump up as you are experiencing. I sort'a doubt that this is your case, but beware of commercial washers, especially if they have a rind of detergent anywhere.
Anyway, I got a good top loader, with no mechanical agitator (water jet only. Front loaders can shake the house!)I use a warm water setting. I use about 1/3 to 1/2 the amount of Nikwax recommended. Then rinse it two to three times. Drying is medium, but check your fabric shell periodically. The heat can damage it. Down is good right on up to 180F, but, the *shell* can be damaged with any heat.
2-3 hours is pretty good with 3 or 4 "dryer" balls. These are short spikey urchin-like thinks than do a bit better than tennis balls. Then let it hang for about a week (at least here in NY.) Do not play with it over much. The clumps will loosten with drying...
My thoughts only . . .