Shared calendars are no longer updating on our phones

[AC Question]

We share calendars and recently, no calendar app works wrt updating or adding events that the other can see. Google calendar app is what we've used but no calendar app works now. But on our PCs, we can update and see the shared calendars just fine, its only when any app is used on our Samsung phones that it no longer works. We've cleared data, caches, synced, tried other apps, etc and nothing works.
Help please!