Birthday calendar don't sync: how to do?

[A-G-X]

I have a problem, i'm using an outlook.com account.

I create a lot of new contact on People on my new Lumia.

I include in every contact the birthday data.

But after that i saved, i can't see nothing on the calendar app As if the phone can't sync correctly online.

How to solve the problem?

I tried to delete and recreate the birthday calendar, but without success

If i go online and refresh the calendar online, the birthday contact appear... but this is not normal!

How to solve?

Thanks