From some reason I can't "mate" this STEP assembly. It clearly shows different parts and I can move those different parts separately, but from some reason when I try to mate, it considers those parts as whole. Here is a video that explains what I'm trying to do:

https://www.youtube.com/watch?v=MnAMGh3Dopk&feature=youtu.be

I will attach the file if you deem it necessary,

Any idea how to address this?

Edit:  I should mention there are no pre-assigned mates in this STEP file.

-Val