How do you add a new part for export

OS X 10.10.1 NB 6d886b7

I have a score with voice, bass, guitar, piano, alto sax, tenor sax, bari sax, trumpet 1&2, trombone and drumset. The first time I created parts, I did not create them for voice or drumset since they were not completed yet. They are now done, but I can't seem to create the parts in the parts dialog.

Attached is a screenshot of the dialog from File -> Parts. You can see all of the parts on the right side of the dialog, and the parts that were previously exported on the left, with the checkbox selected for bass. However, everything is grayed out, and you cannot select the checkbox for voice or drumset, and therefore cannot create those parts. I can select "New All" and create the parts, but then I need to delete the duplicated parts. This doesn't seem like how it is intended to work. Is this a bug?

I have attached the score as well.

Thanks in advance!