Add-captions randomly greyed out - glitch/bug or just user mistake?

Hi,

I am currently captioning a longer video, writing the captions manually. I'm about halfway through the project now and from seemingly out of nowhere, the "add new caption" + sign randomly greys out from time to time, without me at least knowingly klicking someting to make it so. Everytime I re-start the app, the feature works again just fine, but then, again, after a while - it greys out and I have to re-start the app again, which seems excessive. I've never ecountered this issue before whilst captioning in premiere - so I am wondering whether it's some sort of glitch or bug, or if I am actually accidentally greying it out myself. Any thoughts?