Or a kidney?
In my own experience the biggest factor in this is the cartridge. My cleaning routine has never changed (anal bordering on OCD) but with some shells I'm scrubbing away for ages and with others the gun hardly needs cleaning.
Cleanest by far through my tubes are the Clever T2's I've been shooting for the last 12 months or so, which bizarrely leave absolutely nothing behind in the barrels after a normal 100 bird shoot.
Other suggestions are as previously outlined, letting the solvent soak for a while before scrubbing is a good one, but also check your bronze brush is still in good shape. These can wear quite quickly and soon be performing below par.
Hope you get it sorted.