Hello Paul,
I think that the problem is related to the incorrect coordinate information in the Reference pane:
- some cameras have zero values for Longitude and Latitude,
- camera altitude values is lower than marker altitude,
- load XY coordinate for the markers do not fit the selected coordinate system (WGS84) - if you are working with local coordinates for the GCPs, then you should uncheck all cameras in the Reference pane, select Local Coordinates in the Reference pane settings dialog and use only Generic preselection option during Align Photos operation (unless the images have been matched prior to the markers being introduced to the project).