Extension lines not working

I've made about 30 or so files in MuseScore and while making the first half of those, I had no problem creating extension lines (type lyric, press shift and hyphen for underscore). But somewhere along the line, the extension line trick stopped working and now I can't create them at all.

I've gone back to one of the earlier files, and there the extension lines all are, but if I edit this earlier file by re-creating one of the extension lines, the thing disappears and I have to hit command Z to get it back.

I'm using 4.3.2 but the earlier files would have been made in an earlier version.

Has anyone else had this problem? And how did you fix it?
Thanks