If the chamber's not rusted and the extractor still has it's hook, try checking the edge of the chamber where the firing pin impacts and see if there might be a burr. Dry firing can sometimes raise a burr on the chamber edge that can overcome the extractors grip. It may help to know the model of he rifle in question also... You might also check the recess in the breech face for the extractor to clear and be certain it has not become filled with gunk(note the technical term!!).