Make sure you torque enough & use carbon paste.
If it's a tolerance thing, then consider using a soda can shim, or some fiberglass cloth.
Also consider greasing the thread of the plug, especially if you've sweated on it. The torque shouldn't go into turning the screw, the torque should go into pushing the expander into the seat post.
Afaik, save for crazy exceptions, "all" seatpost slipping is fixable.