I've subsequently found that this may have been a file problem. the subproject features work after I've set up some new .rmp project files.

I still think you should look into it: I originally formed one big project file including both alpha and beta projects, then created separate files by copying the big one to 2 places and eliminating alpha tasks from the beta file, etc. Whatever the cause, Open Workbench refused to deal properly with the two subprojects.