Make sure to clean any washer you use fairly thouroughly. The residual detergents in commercial washers can start causing problems with down. If you cannot find a clean one, a good rinse with lots of water...especially the soap bin, may work to clear any detergents/strong soaps out of the machine.
Yeah, more washing would be my first guess. Generally, washing does not hurt down provided the soap does not strip the lanolin like oils out of the clusters. A very mild soap, downwash, woolite, and the like will work. Water does the majority of the washing. So, I usually use 1/2-1/4 as much soap as needed. I usually recommend 1/2 for the uninitiated. In this case, I think I would do it twice, with a normal rinse cyle between, then an extra wash cycle(no soap) with extra rinse. IE wash it three times: twice with soap, one with just plain water. Dry it thuroughly with 2-4 tennis balls. As you say, if it is already degraded, no harm done. If it is good, but oily, this should restore it.
If it comes back plan on washing it every 14-21 days with down wash. Again about 1/3 as much down wash as is recomended. An extra rinse wash to insure thourough rinsing. Good down, 800fp, requires a bit more cleaning to maintain the higher level of performance than cheaper 600fp down. It will flatten out/clump quicker with body oils and sweat than cheaper down. Note that eventually all soaps will degrade down's performance. So, use enough, but don't over do the soap. Even plain water will eventually degrade down, but this will take many more years than you have on your bag.
Unfortunatly, if any prior washings were done with any detergent, or the bag was dry cleaned (some of the solvents are fairly harsh with down,) it *might* be too late to help the bag. Clumping is also a sign of degraded down due to incorrect laundering. But, as others have said, it should not loft back if it had been badly degraded already, at least not consistently.