Unlike standard forms, popup forms are independent of the Access application interface.
They will always open on the same monitor where they were last saved.
If that was done on the secondary monitor, then the form will disappear next time you use a single monitor as it is 'appearing off screen'
So to force them to open on the primary/only monitor, open in design view with Access on the primary monitor.
Save the form and it will be visible when you next reopen it.
This isn't an issue with standard forms as they always open as part of the Access application interface.