+1 on the release valves. I'm building a de dtijl up for someone at the moment and it's missing a valve completely. For now, I taped over the whole area to make the machine useable, and the suction was dramatically improved just from doing this
You do have to be careful not to block the machine up though with the tape in place.
There is a seal that sits on the chassis of the machine (non removeable). Clean this up until it's black and squeaky again, and the mating face on the cyclone. There is an internal seal inside the cyclone, but I doubt that has been dislodged.
If you take the cyclone off, and remove the clear bin, there is (should be) a seal where the clear bin meets the cyclone top. Does it work any better with this clean? With the machine on and the cyclone on, if you push the bin up into the cyclone assembly (to push it onto this seal) is there any difference?
With the cyclone fitted, is there a difference in motor tone compared to the cyclone not being on? You may have to listen out for/block the release valves to be sure though, as these could open if there is a blockage...