Just throwing this idea out there...
What if you stopped and started the vacuum every couple of minutes with it attached to the DD.
I say this because I have noticed that when I'm sanding to a finer grit (150, say, or higher) that there is a permanent 'tornado' going on in the upper part of the DD cyclone that never falls. If I turn the sander off, the vacuum goes off and the dust falls into the box.
Maybe if you didn't run the sander continuously and let it turn off once in a while, the smaller dust particles would do the same.
Love the DD, but haven't put it through its paces yet on finer particles. I have the 'Ultimate' setup, which makes the bulk at least a little more organized (the box that the dust falls into fits onto the 26/36 and clicks in as a unit). It's an expensive attachment, and I thought about returning it at first. After a couple of months, though, I really like having it. The vacuum is still has scary suction and my first bag is less that 5% full. I've emptied the box twice with more than enough stuff to fill a bag. There are less expensive options, but space is a commodity and I like having the whole unit as an integrated system.
I guess there's nothing about this Festool system that's cheap. [embarassed]