Your Outlook calendar subscription isn't updating
Like Google, Outlook fetches subscribed internet calendars (ICS URLs) on a server-controlled schedule you mostly cannot change, typically every few hours up to a day. If the feed is fine and Outlook is behind, it is the refresh cycle. The wrinkle specific to Outlook: classic desktop Outlook, new Outlook, and Outlook.com each sync these subscriptions differently.
Work through these, most likely first
1The refresh interval
Outlook.com and new Outlook fetch subscribed calendars server-side every few hours at best; there is no manual refresh. Classic desktop Outlook honors the send/receive schedule, so a manual Send/Receive All can pull an update.
2Subscribed in one Outlook, looking in another
A subscription added in Outlook.com lives in that account and flows to the apps; one added only in classic desktop Outlook stays local to that profile. Add URL subscriptions at Outlook.com (Add calendar > Subscribe from web) so every client sees them.
3The feed rejected Outlook's fetcher
Outlook's servers fetch the feed from Microsoft's cloud. Feeds behind logins, IP restrictions, or aggressive bot protection return errors to Microsoft that you never see, and the calendar just stops updating. The feed must be publicly reachable by URL.
4The URL changed
Outlook keeps showing the last successful copy of a dead URL indefinitely. Verify the link still serves ICS, and re-subscribe if the publisher rotated it.
Rarer, but worth knowing
Work accounts. Microsoft 365 admins can disable internet calendar subscriptions entirely. If the Subscribe from web option is missing, that's policy, not a bug.
TangoHive feeds are public, stable URLs designed for exactly this kind of fetcher, and every calendar also has a browser share page that never waits on Microsoft's refresh queue.