The drum has to be very very clean and the print emulsion has to be hardened, either with the acid additive for the rapid fix or a Chrome alum bath, there may be others. Another thing that was commonly used was a ferrotype wax that was put on the metal to help shine up the gloss and enhance the separation of the paper from the surface. It's such a pain to get good results most folks just don't bother, but when it's good you get real snap to the image.
BTW I have one of thos 1620s, never use it anymore.