There seems to be a constant issue with (especially CF, particularly the %@$# Alien ) seatposts in Calfee frames.
I was plagued with this - the first few rides on my Dragongly I was stopping every 10 miles to raise the seat and tighten even more.
Finally I cleaned out the seat tube with a cloth moistened with rubbing alcohol. Until the cloth came out clean. Then I cleaned it some more. Cleaned off the post also.
Since then, if I ever have to remove the post I repeat the procedure.
No more slipping. My conjecture is that the carbon dust was acting as a lubricant.