calendar entries made on iphone do not sync with office 365. The other way round works. Any clues to solve this?

I have an Iphone 6 plus fully updated. I use for mail, calendars and contacts an office 365 account. It syncs perfectly except for entries made on calendar via iphone. It does not appear on outlook (not even on the website). The other way round Works fine. Any clues to solve this? I have already tried the following:

Delete the account and set it up again.

Disabele calendar sync and enable again