Can you use Clash Detection on nested references in OPM?

I set up my Projects using nested references, divided by discipline:

Example:

I) Project.dgn

        1) Arch.dgn

              1a) A-BLDG-1.dgn

              1b) A-BLDG-2.dgn

        2) Elec.dgn

             2a) E-TRAY.dgn

             2b) E-CONDUIT.dgn

        3) Mech.dgn

        4) Struc.dgn

etc.....

I'm trying to run clash detection, but under the References tab, I only see my active file, and the top-level Project.dgn file that is directly referenced into my active file.

Is it not possible to run clash detection on nested references?