I've just recently started using ArcGIS for Autocad 400. I am trying to load our companies services. The issue I have is that I can get some services to load in some maps, but not others. Settings seam to be the same (Coordinate System, Units, etc.). I'm not sure if there is something I could look for that would be preventing services to load in some of my maps.
Are you working with map services or features services?
In the case of map services it could be that the services that fail to add are tiled map services that are currently unsupported by ArcGIS for AutoCAD.
If you are working with feature services check your project area, it could be limiting what features are drawn in the drawing.
Hope that helps, let me know if not.