Quoting michaeldavishome,
Cheers for the reply - unfortunately, that didn't fix it for me
Mike, I am going to need more help \ feedback than you are providing...
Are you able to see the Secondary when not using Advanced mode?

If not, that's the problem that needs solving 1st.
If you do see it, when you switch to Advanced, does any zoning out \ moving show it? Example:
https://cdn.stardock.us/support/uploads/MultiplicityConfig_2025-05-29_09-47-33SeanDrohan.mp4
If you try to add the Secondary again there, does it then show?
Are you certain you are using the EXACT same version of MP4 on each PC?

Sean Drohan
Stardock Product Lifecycle Manager