You have fouling as others have said.
You don't need a brass brush or a Lewis Lead Remover.
Cut about a 1" square from a new piece of Scotch-Brite (the green scratch pads sold in the Supermarket for scrubbing out pots & pans). Use this to clean the chambers by pushing it through with your cleaning rod. Solvent is a help here.
Next time, inspect the chambers and the bore by looking through them at a light or at a piece of paper held to catch the light. You will see whatever crud has been left behind by ineffective cleaning.
Also Remington blitz cloth will remover fouling including metal, quicker than just solvent and a patch. Don't use it on blueing tho as it will remove it if used very hard.