If it has an ebonite or vulcanite stem, heat the stem (with the stem removed from the pipe) with a heat gun (or hair dryer) and then let it air cool back to normal temp. The material will expand with the heat, just enough to re-tighten the fit of the tenon.
Not sure where I read this from, but it works, I have done it twice with good results both times.
It is important to wait until the stem is cool before fitting it back into the pipe.
Evidently the ebonite or vulcanite dries and shrinks over time and upon heating, it returns to it's normal shape or condition.
Good luck!