On you phone, go to Settings>Mail,Contacts,Calendars>Default Calendar and make sure you have selected a calendar listed under your iCloud account as your default. If not, make this change, then enter a new event on your phone and see if it syncs properly now.
If your settings are already correct, or if after changing them it still isn't syncing properly, make sure all the calendar events on your phone are in your iCloud calendar. If not, enter any missing events either on your iPad, or on icloud.com from your computer. Then, after your iCloud calendar is up to date, go to Settings>iCloud on your phone and turn Calendars to Off. When prompted, choose Delete from My iPhone. After a minute or so, turn Calendars back to On to sync your iCloud calendar back to your phone. Then try adding a new event on your phone and see if it syncs properly now.