I have been having the same problem, I randomly don’t receive notifications.
I have noticed it happens more often when my clock says “weekend”. I’m not sure how it works, since even during the weekends both me and my opponents make moves in our games. I will log in sometime on Friday, play a move and then log in on Sunday and see my opponent has also played and it’s my turn. But I never got a notification.
It also happens during the week, maybe I paid more attention on Sunday because I’m not at work.