This guide explains the various ways to troubleshoot your subscription calendar.
Events from the subscription calendar are not showing on my Apple device (subscribed using Google Calendar).This is typically for users who have subscribed to a calendar using Apple (desktop) or Windows (desktop) and is using Google Calendar.

Events from the subscription calendar are not showing in the Google calendar app (Other Calendar services too like Outlook, Apple, Yahoo, etc.)Make sure that the subscription calendar is checked on the user's Other calendars section. If the calendar is unchecked in the list, they should check it. If it does have a check then they need to re-subscribe again...