How to fix Calendar alerts not working on iPhone and iPad?

When I send a family member with first and second alerts, they don't get them on their end. The same happens when they send my one. I get the same result whether I am using my phone or iPad. We have devices across the spectrum with similar results. All iOSs are up to date.

It used to send and receive the alerts. It has become an issue with more activities.

[Re-Titled by Moderator]

Original Title: No calendar reminders when I send or receive invites with them to or from others.