Difficulty Adding Staffs

For our choir, I and create an mp3 file for each part to help members practice. I scan the hardcopy music to pdf, open it in Capalla Scan, recognize it, clean it up, and export it into musicxml format that MuseScore can read. I have one of those SATB songs that I am now cleaning up in MuseScore (attached). The SATB songs i've seen typically have one staff for Soprano & Alto and one staff for Tenor & Bass. I usually add staffs for the Alto and the Bass so that I can explode the Women's and men's parts into all four SATB . This piece came with some of the parts imploded and some already exploded. When I try to add a staff to explode the combine parts portion, they do not show up on the screen. Can you help? Thank you so much.