Consider this full score layout. This is an old music XML file imported from Finale and I can’t seem to coerce it into squeezing more systems per page no matter how much I adjust the spacing settings. I don’t have this problem with any other files so I’m attributing this to the import (but it may as well be my own ignorance!):
Ah yes! Once I saw your message I remembered that I went through this exact same issue with other imported files. I selected all frame breaks, deleted them, and all of a sudden the scores flows normally.
Thanks everyone else as well for your suggestions!