Why do two smooth, clean-cut lead sticks stick together when pressed together and the chalk pieces don’t stick together?

Intermolecular forces of attraction manifest themselves only at distances commensurate with the diameter of the molecules. Lead can be polished to such an extent that attractive forces of molecules begin to appear between the cuts of the lead bars. Chalk is a rather brittle material, and the distances between molecules remain too large for the manifestation of attractive forces.