Keep an empty shader slot open, for some reason my shader slots were always full on my chars and i never got drops even though they are supposed to go to the Postmaster. But i've found after TTK, all my missing rare shaders by simply leaving a slot open...hope it helps.

I just got that last night and substance 321 a few days before. I didn't even realize they were somewhat rare until my friend started freaking out when I told him lol