It all comes down to using the right approach in the first place.
Theory
The primary goal of down care is to preserve the fluffiness of the feathers. There are three enemies to it:
- Dirt: fat and salts coming from your body
- Breaking: from mechanical action, such as washing too hard
- Lumping: from improper drying
By washing, you took care of (1). By using uncompressed storage and delicate washing and drying programs, you avoid (2). (3) requires the proper technique and action on your side.
Recommended technique
All suppliers I know of do say that washing from time to time is recommendable; if done properly, it should not do any damage (still, I agree with @fgysin that you should wash it as rarely as possible). You didn't do a bad job IMHO -- Mountain Equipment¹ recommends² 800 rpm delicate washing at 30°C30 °C in a large machine, and special down detergents.
Now, the drying is actually the more intricate process. You need, again, a delicate program, a large dryer, and low temperature (but this is more important for the plastics than the down). Alternate a few passes of careful machine drying with with some tennis balls (or other soft clean objects) with airing the bag (for days!). Shake and fluff it between passes. Several passes may be needed; at least two, in my experience.
The secret
Now comes the manual part. Take your dry but lumped bag to some passive, sitting activity - listening to music or watching TV. While you sit, tear apart the lumps in each segment and redistribute the feathers. Shake the bag, repeat. For at least an hour.
This manual treatment is what I recommend you try. It really is possibly to re-fluff a completely lumped bag into a state that is better than before washing, with enough patience.
Last resort
Mountain Equipment recommends not to use them, but there are services for professional cleaning and refilling sleeping bags. They will remove the down, wash it, optionally top it up a bit, and put it back in. This should, in case your down is just badly lumped and not broken, fix the problem.
Another thing that came to my mind is the design of your bag. Modern bags are organized in segments, sometimes even overlapping ones, so that down can't all coagulate in one corner at any time. However, older models may not have them and be organized as just one chamber, like a blanket. In that case, more than shaking, there's really nothing that can be done except buying a new bag - but you might still be able to recycle the old down.
¹ I mention them a couple of times simply because I found their descriptions best - this is not an advertisement, nor am I affiliated with them.
² There's an English version of the same page, but I find it worse than the German one.
