I’ve noticed that if I leave the screen on an app (YouTube) overnight till the morning - I’ve got a blocking session from 6-9 AM for 3 apps including the one I just mentioned - Opal doesn’t start the session up until I turn the app down and run Opal app.
In short, if you are using an app before the session starts, it won’t start, at least that’s my suspicion.
Edit. I’ve recorded a video how it works but unfortunately can’t add it in here, if you will need it let me know where to send it