First determine the direction of the delay: event created on the web but missing on mobile, or the reverse. Create a neutral test event with no personal details, then note when and where it appears.
Include the calendar provider, client and version, approximate local time, elapsed delay, and whether the same account is signed in on both devices. A test event that syncs while older events do not may point to a narrower issue than a complete sync outage.
Where was the event created, and which device is still waiting for it?