Unable to update txml source file from external review

I have used a txml file as source in sdl studio 2014.

File was pre-populated source -> target and then exported to bilingual Word for translation outside Studio (unavoidable workaround).

I now have my translated MS Word file. When I select the Update from external review process, the source txml file does not appear in the panel to update. (opening the file in the editor and going directly to Review-> Update from External review also does not offer the updating workflow).

Would anyone have any idea why this would be? (the source txml appears without problem in the Editor and Files panel themselves) - it simply is not there in the update from external review pane.

thanks in advance for any advice offered.