Welcome to the Apple Community.
Please try the following…
- First check that all your settings are correct, that calendar syncing is checked on all devices (System Preferences ➔ iCloud ➔ Calendars ➔ On on a Mac and Settings ➔ Apple ID(Top) ➔ iCloud ➔ Apps Using iCloud ➔ Calendars ➔ On on an iPhone, iPad or iPod).
- Make sure the calendars you are using are in your 'iCloud' accounts and not an 'On My Mac', 'On My Phone' or other non-iCloud account (you can do this by clicking/tapping the calendar button in the top left corner of the application ), non-iCloud calendars will not sync.
- If you are sure that everything is set up correctly and your calendars are in the iCloud account, you might try unchecking calendar syncing in the iCloud settings, restarting your device and then re-enabling calendar syncing settings.