Adding Solo Line Back Into a Song?

I'm working on an a cappella arrangement with one of my friends, and he created the document I'm editing. The first part uses a solo line, the last chorus does not. The score is set up with a treble solo and SATB chorus, all made up of piano lines (two treble piano lines, one grand staff piano line), because they're easier to hear for us during rehearsals. When I clicked add measures, the resulting measures all lacked a solo line, and I cannot seem to add it back in. I tried using the edit menu and adding a treble piano staff, but it did not do anything. Am I missing something really obvious? Please help me, this is driving me crazy.