No luck with a vacuum cleaner?

It's worked for me a few times!
A homemade vacuum attachment with a smaller opening might work. Roll up paper or cardboard into a funnel shape such that the new nozzle is a little smaller than the dust cap. The nozzle end needs to be even. Strong tape might also work, but I wouldn't use it on a paper dust cap.
If it's made of paper and the vacuum attachment doesn't work, I would poke a pinhole(s) in the dust cap. Then bend a right angle into a thin piece of wire about 1mm from the tip. Insert the wire into the hole and pull out the dent. Fill the hole(s) with a small drop of epoxy. If you have weak creases in the paper, gently smear a THIN coat of epoxy over the dust cap.
Thomas