My Jet 1014 has a Nyloc nut on the threaded rod the washer is fitted over, and no matter how much the washer ever turned, it is not going to get any looser --or tighter-- than where I set it. Mine locks rock solid.
The problem i have is that when a long mandrel is inserted into the tailstock and aimed end-to-end at a mandrel or dead center in the headstock, they are misaligned by maybe 0.010", to the side AND up and down also........I contacted Jet, the tech guy there wanted to send me a new quill assembly, I tried to tell him that the problem was NOT in the quill assembly but in the alignment of the bottom of the tailstock itself.
He sent me the quill anyway, after installation it measured exactly the same misalignment as the old one, so I still have a lathe that is misaligned. Re-machining the bottom of the tailstock would be the only remedy I can see.