Quick answer
Calendar not syncing is four different problems wearing one name. Either device sync is off for that account, or a subscribed calendar is just waiting on its refresh cycle (Apple checks about hourly, Google every 12 to 24 hours), or a shared calendar was never accepted or is hidden, or the source itself is broken: a dead link, a regenerated URL, or a calendar file that was imported once and will never update. Identify which one you have before changing settings, because the fixes do not overlap.
Calendar not syncing is four different problems wearing one name, and the fixes do not overlap. Changing settings before knowing which one you have is how an afternoon disappears.
The one-minute triage: a single calendar missing everywhere points at the source or the sharing, everything stale on one device points at device sync, events that arrive late but do arrive are normal refresh lag, and events that never arrive mean the source is broken or was imported as a file.
Device sync is off
The common one on phones, and the quickest to check.
On iPhone: Settings, Calendar, Accounts. The account has to be listed, and Calendars enabled for it. Then check the account's calendar list, because iOS happily syncs the account while individual calendars stay unticked.
On Android: Settings, Accounts, pick the account, Account sync, confirm Calendar is on. The Google Calendar app has its own layer: open its Settings, tap each calendar's name, and confirm Sync for the specific calendar. A calendar with that toggle off vanishes from the phone while the web version looks fine.
In Outlook: confirm the account is connected in Settings, and the calendar's checkbox is ticked in the sidebar. A hidden calendar and a broken one look identical.
The subscribed calendar is just slow
A subscribed calendar updates on the app's schedule, and the app does not ask your opinion. Apple checks about once an hour, Google every 12 to 24 hours, Outlook every few hours. A change made this morning reaching the phone tonight is the system working as designed, not a bug. There is no button that forces Google to check early.
If a full day passes with nothing: open the subscription link in a browser. A page of calendar text means the source is fine and the lag is real but normal. An error page means the source moved or regenerated its link, and every subscriber is stale until they re-subscribe with the current one.
The shared calendar was never fully accepted
Three switches have to be on at once: the invitation accepted, the calendar's checkbox ticked on the web, and its sync toggle on in the phone app. Any one of them off looks exactly like sync being broken. Walk all three before concluding anything.
The source is the problem
Two failure modes here. The link died: the organizer's tool regenerated it, the account behind it closed, or the page hosting it moved. Re-subscribing with the current link is the only fix, and every subscriber has to do it.
Or there never was a link. A calendar file that arrives by email and gets opened is an import: a snapshot of the events as they were that day, with no connection back to the source. It will never update, and the next email with a corrected file starts the cycle again.
Both are the same disease: the schedule has no stable hosted link. That part is fixable. Put the schedule behind one link that updates in place, Calfeed hosts exactly that, and subscribers stay current on their app's normal refresh cycle. It will not make Google check faster than 12 to 24 hours, nothing will, but the link stops dying and the email-a-new-file cycle ends.