Yeah maybe trimming it down just a bit would help, I listened to the stove shop years ago when they said "it should loosen with use" but that was 25 years ago and I'm still waiting lol!I wonder if you gave your brush a little trim might be as simple as chucking it up in the drill and running it on some 60 grit sand paper .015 -.020 might make a big difference in resistance while still cleaning the chimney fine.
or spend the coin and get the rotary cleaning brush and chuck it up in the drill , that would be sweet , I just can't justify the money when I have a setup that works and gets such little use.