When I have this problem with my 617s & 63, I clean thoroughly using a NEW brass brush. When the patches are coming out clean, I run some lead-away patches through (cut to the size of 22 patches), let set for a few minutes, then run more cotton patches through. Works wonders. I'm always amazed at how much additional lead the lead-away patches will remove.
This is what I've learned to do with mine. The lead wipe away cloths will clean out gook I/you never knew was there. No power tools, just a small patch, slotted tip, and a cleaning rod.
Follow up with a clean patch/dry patch with your favorite solvent. Repeat if necessary.