I am trying to mate one subassembly 'A' to a fixed face in assembly 'B', but I cannot select the desired face of 'A'. When I click on the face to mate, it simply selects the face of 'B' behind 'A', as if it isn't there at all. The select other tool is no help. 'A' does show up in the feature manager, and can be selected there, but I cannot select any faces to mate in this way, only parts in 'A' or the whole subassembly 'A'. I saved, rebuilt, and reopened 'B' with no change in results.
The one unusual thing that I can think of about the subassembly 'A' in assembly 'B' is that it was added through a master assembly 'C', where it 'A' was moved on the feature tree from the top level into the assembly 'B' folder like so: