Partial Object Recognition

I'm trying to get Vuforia to recognize a specific part of a larger rig meant to hold another object that, due to it's specular nature, Vuforia cannot recognize alone.

So far I haven't had any success scanning the rig despite its relative simplicity (three uniquely colored spheres at the end of ~two inch long arms sitting below the mounting point for the specular object).

I'm hoping that someone might be able to help me identify which part of the scanning process I'm doing incorrectly, or where I might make modifications to increase the accuracy of identification.

Thanks, much.