Lack of Cloud 'cross-copy'

Go to Settings>Mail,Contacts,Calendars>Default Calendar on your iPad and be sure you have selected an iCloud calendar as your default and not something else like On My iPad, a Google calendar, etc. If you haven't, make this change, then add a new event on your phone and see if it appears on your calendar on icloud.com now.

If your settings are correct and new events added to your iPad still aren't appearing on icloud.com, add any missing calendar events from your phone to your calendar on icloud.com. Once the calendar on icloud.com is up to date, go to Settings>iCloud, turn Calendars to Off, choose Delete from My iPad when prompted (your calendar will still be in iCloud), then after a minute or so, turn Calendars back to On. Then add a new event on your phone and see if it appears on your calendar on icloud.com now.