Strange iCloud Calendar Sync Issue (iOS 9.3.1)

[ndriver182]

I noticed recently that the calendars on my iPhone and iPad seemed more empty than usual. I found for some reason that certain calendars are not transferring over to my iOS devices. If I log into iCloud online and check my calendars there, everything is fine. If I check my calendars on my PC using Outlook, everything appears to be there (as far as I can tell). Why on Earth would some calendars sync over to my iOS devices, but not everything? To be clear, I DO have only one iCloud account and it's enabled on all devices. I have made sure my devices are logged in and all calendars are "enabled" inside the calendar app. iCloud is definitely also set up for syncing calendars in the iOS settings as well. I've also tried logging out of iCloud in iOS and logging back in, but that didn't fix anything. I've never seen this before. Any ideas?