Have you lost your "stopper"? Here is a replacement. Available in nickel, brass or black.
These absolutely must go priority mail, I know.. it's expensive, but these little things get eaten by the post office machines when sent in a small envelope and this is the only way to safely get them to you in a timely fashion.
Pro-Tip: Once adjusted, a dab of thread locker (Loctite etc) will keep them from budging. Purple Loctite is removable, the blue and red both need heat to free them. (hint: Gasket goo or silicone caulk will work as well, just a dab will suffice).