Why is iCloud not syncing my calendars on my iOS devices?
Your iOS device may be saving new calendar events to a local calendar instead of the iCloud calendar. This is particularly confusing if that local calendar has the same name as the calendar you want to use in iCloud. To change the default calendar for saving events on an iOS device, navigate to Settings > Mail, Contacts, Calendars > Default Calendar and select your preferred  calendar under your iCloud account: