Shared iCloud calendar events not updating on shared devices

I had successfully set up a shared iCloud calendar with my friend which enabled us to edit and view the same calendar on our iPhones and iMacs.

Everything was working fine until a few days ago when it stopped working. When either one of us created new events, they wouldn't show up on the other person's calendar and when they seldomly did, not even an alert popped up like it used to which the person had to go under the invitations page and accept them.

What could possibly have gone wrong? We tried deleting the calendar then re-sharing but it stopped working very suddenly.