AFAIK the coil zippers are always plastic, so even if the slides are metal, there's nothing for them to stick to. They may be jammed with something, or possibly they are pot metal that has expanded from corrosion
so badly that they can't be run on the zipper anymore. So I'm going to guess PBlaster or something else that will break down any oxidation blooms that are jamming them, with a good chance that it it simply time to replace
them. Either with brass slides or plastic ones.