J bent piping doesn't stay tight
I just removed my bathroom faucet. I have a metal pipe coming from the wall with no threads on it. I went to the depot store and they gave me the j-bent pipe with washer. However when I start assembling the tailpipe and the rest of the parts the j-bent pipe starts to loosen from the wall to the point that it loosen with just a touch. I don't want to assemble the whole thing only to have the j-bent pipe loosen up in the end. I have used medal to medal, medal to plastic. Any suggestions would be greatly appreciated. What am I doing wrong?