Depleted Artifact needed to complete Under Mountain

jjmm4

I have been in multiple HE's in Vanrakdoom, Slippery Slope and all of the UM maps and i never receive the Depleted Artifact. Can you increase the drop rate for the depleted artifact? i had this same issue on another character and it took a really long time for it to drop. It's should drop as easily as gathering the resonance does.