Insert Clef/Instrument Not Working

I am nowhere near a MuseScore super user, but I know enough to work with it. I help my choir out with drop octaves and various part tracks using MuseScore. For some reason on these newest songs I import (.xml from PlayScore2 scans) it is not letting me add a staff/clef.
In the attached file I am simply trying to add an additional bass clef below the current one, so I can copy-drop the octave on the Tenor staff. I hit the "I" key to insert > Vocals > Bass > arrow, just like I always have. It creates a new Part and Mixer track, but the staff is not there. It looks like it shifts the measures over a bit, but there is no Staff/Clef added.
Am I losing my mind? What am I doing wrong? I can open previous scores I have edited and they add staffs just fine using the same process. Help please.