There is something wrong with preview panel. It sometimes doesn’t show up. Or just doesn’t refresh. Or something worse.
What do you do when it doesn’t show up? The only case I can think of is when your file is multiple master and the outlines are incompatible. Is it not the case?
Which version of app, OS?
i had the same. newest beta and 10.7. the preview panel itself was there but it was just plain grey. i restarted glyphs and it was ok.
Version 1.4.3 (581)
I think i fixed it.