We're fortunate in having a Kodak rep who takes ownership of problems like this. We recently had an issue with spots on a big job, long run and many plates, as usual, finger were pointed at Prepress before looking into anything else, I called in the Kodak rep, who organised a free pack of plates from another batch, the problem persisted. He cam back with a microscope to look at the spots and suggested they were coming from the press. He then opened a palette of the stock we were using and went through it sheet by sheet.