well if the other tubes fit fine it sounds like the one tube is the problem. this sounds simple ,but put solvent on the tube and on the threads in the barrel and rub with a fine wire brush or even a stiff tooth brush.wipe clean . sir carries choke tube lube, but any light oil will do. apply to your choke tube and see what happens. as some one said , chuck em and buy another at your local gunshop if nothing else. it doesn't sound like a barrel problem