Premiere Pro 2020 subtitling on Mac High Sierra 10.13 is not playing ball

Although I've managed to create open captions that work fine in a couple of videos, now starting a new one, I've got stuck. I wanted to try closed captions but they didn't work, even after enabling them in the program screen. So I tried changing them to open ones, but even though I can type the words into the boxes, and format them with the font I want, and the lilac strip appears along the top of the clips, and the little black dialogue blocks are there with the text in them, nothing shows on the project itself. It's so tiring and I'd be happy just to use open captions for now.

Someone suggested using an app called Subtitle Edit but I'm not sure it's available for Mac and hope that it's not necessary.