... Developers usually focus on UID and SEQUENCE. Google checks more than that. An update will only apply if all of the following match Google s internal model: The UID matches an existing event The SEQUENCE value is higher The VTIMEZONE block matches the original event exactly DTSTART and DTEND changes pass Google s normalisation rules STATUS, ATTENDEE fields and alarms do not conflict with the stored event If any of these checks fail, Google usually accepts the file but ignores ... ... difficult to reason about. 4. Ways to reduce the chance of silent update failures These steps are practical and consistently improve results: Always increment SEQUENCE Even if the change is only a description update. Keep VTIMEZONE