Calendar won't sync across devices

Hi, so I have an iCloud account, which is supposed to sync the calendar across both devices. On my iPhone, all calendars are shown. On my Mac, a subscribed calendar and a calendar on my iCloud account are not syncing. What can be done about this? Thanks!

On iCloud.com it shows that "the subscribed calendar and a calendar on my iCloud account are not syncing". Is it an issue with the iPhone?

Thanks,

Adam