format one line of text in motion

I'm creating scrolling credits in motion for a fcpx project. I know that in theory I'm supposed to be able to change the formatting in motion for a single line of text (one line centered, next 4 lines in columns, then a centered line again, or giving the "role" a different font style than the name), and I've done this before, so I know that it works.

However, it seems to have stopped working for me on this project! I copied and pasted all the text from textedit into motion, and it wouldn't let me adjust the justification on the "subject" headings, then a little while later it started to work great (no idea why), and then it stopped working again and I can't make any changes to a line without changing the whole document again.

Any ideas why? Is there a setting I've accidentally changed somewhere?