Greek z and IE *j

Diverging from recent topics and going back to a subject more typical
 of earlier threads on cybalist, here is my question:

Why does IE initial *j appear sometimes as Greek <z-> and sometimes as
 Greek <h->?  I delved into the archives but could not find any
 satisfactory discussion of this problem, mostly by-discussion that produced
 theories for which there was contradictory evidence.  Can anybody offer a
 solution now?

Andrew