Subtitles Won't Display

Using 4.20, in my main project, subtitles from dialogue wave won’t show up. I can create a new project and they work just fine, so my question is what might cause this? How can I debug what’s going on? I’m not getting any errors related to the subtitles and I’m using the default Roboto font, the audio plays fine - it’s just that the actual text isn’t displayed on the screen? Any suggestions?