I'm trying to get full rathian but only the tassets and helm are showing up even though I have all the stuff needed for the full set. How do I unlock the mail, leggings and gauntlets?

for mail you need ian scale, shell, machalite ore and monster fluid; for braces the same as mail plus lg monster bone without monster fluid; for greaves same materials as for mail plus iron ore without monster fluid. I think you don't have some materials from that i wrote above.