Material melding item levels

[Bixby]

I was attempting to meld a craftsmanship +6 materia, to my Alchemists head piece, which didn't currently have any materia. But was being told i'd only get +4 from it.

Yep. The HQ head pieces have caps (for additional stats from materia) of 4 Craftsmanship, 5 Control, and 0 CP. See here.