This is a repeating issue for me, in one way or the other. Most of my issues went away after a new version of Mylio and a reset of sync (got terrific support from the Mylio support!).
This time I have an even about two weeks ago which Mylio refuses to recognise. I have a few photos from this event and I have even deleted the event in my calendar and created a new one, but to no avail. I’ve checked the time of both the photos and the calendar event and it does match (the event was about 2 hours long and the photos are from in the middle of it). Events a couple of days before and after this event are imported correctly.
It is a bit annoying, anyone that has any suggestions? I could create the event in Mylio but that would be a bit of cheating I think, and it would most definitely not solve the issue.