I have the same issue- I cannot get results for items I KNOW are in my calendar. Some items come up on search but the majority do not. The seems specifically for events in the past where some come up in search and some don't. If I do a spotlight search, the items DO come up under calendar items, so it seems like it is just the search mechanism in iCal itself. What's also peculiar is that I can see the item I am looking for in the "suggestions" as I type... but it can't be found.
Also note, I recently upgraded to Sierra, so if this is a bug, Apple STILL has not fixed it!