Google doesn't sync my subscribed ICS feed

I have a ICS feed for a calendar, that I subscribe from Google calendar. As soon as I add the feed, it shows all the events perfectly. But when I edit events, it doesn't update. So it seems like it can read the feed, but doesn't sync it.

If I remove the feed and add it again, its still not synced. But if I add a parameter to the url (ex. &x=1), it will import the updates, but still not sync. So it seems like Google is caching the feed pr. url, but doesn't sync after the url has been added.

Anyone else had this issue, or know a solution?

SOLUTION UPDATE: 12/03 - 2020

I missed the fields "SEQUENCE" and "LAST-MODIFIED". Credits to @anmari for spotting the error.

The calendar now syncs perfectly with Google.

Example from the ICS file:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//ZContent.net//Zap Calendar 1.0//EN
CALSCALE:GREGORIAN
METHOD:PUBLISH
BEGIN:VEVENT
SUMMARY:Gudstjeneste - prædiken ved Enok Sørensen
UID:1177
DTSTART:20190915T073000Z
DTEND:20190915T103000Z
DTSTAMP:20190907T164050Z
LAST-MODIFIED:20200311T195313Z
SEQUENCE:2
LOCATION:Nørregade 13, 6900 Skjern
URL:https://www.lm-kirken.dk/kalender/gudstjeneste-15-09/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Fredagsfællesskab
UID:1248
DTSTART:20190920T153000Z
DTEND:20190920T180000Z
DTSTAMP:20190909T201436Z
LAST-MODIFIED:20200312T062507Z
SEQUENCE:1
LOCATION:Nørregade 13, 6900 Skjern
URL:https://www.lm-kirken.dk/kalender/fredagsfaellesskab-20-09/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Gudstjeneste - prædiken ved Leif Bach Kofoed
UID:1214
DTSTART:20190929T083000Z
DTEND:20190929T103000Z
DTSTAMP:20190909T185328Z
LAST-MODIFIED:20200311T210357Z
SEQUENCE:1
LOCATION:Nørregade 13, 6900 Skjern
URL:https://www.lm-kirken.dk/kalender/gudstjeneste-29-9/
END:VEVENT
END:VCALENDAR

Solution

Bjorn to tell any calendar application that an event has been updated (so that it will sync it) one needs fields to be updated for the event that is being modified, and add:

LAST-MODIFIED:19960817T133000Z SEQUENCE:1 (of update)