My typical path to solve a Revit issue is to describe my issue and search online. 9 times out of 10 that gives me the solution. Didn’t work this time.
My Situation: My AutoCAD Revit 2021 model “A” has a linked Revit model “B”. Inside B is a linked IFC model “C”. In A I couldn’t see the content of C, and it was very important. I looked in all the normal places of visibility graphics/phase/etc. They key to the mystery was to go into Model B and set the IFC link Reference Type to “Attachment”

After a reload, this solved my issue.

I searched for “revit ifc model in linked model not showing” & “Revit Nested IFC Model Not Showing” but neither helped. Hopefully this helps someone.