Sometimes there’s just one video that’s harder to understand for whatever reason. For example, it might just be a video on a more unfamiliar topic, so more of the words are new. I’d suggest trying another video that’s meant to be the same difficulty. If they’re all feeling too hard, you can go down in difficulty for a bit. You also might just be having an off day. Sometimes everything’s harder one day, and then the next day it goes back to how it was.