CRM Parent and child cases mapping

Hi Guys,

There're some fields in child case not being mapped from parent case.  I did the settings at Service Management\Parent and Child Case Settings, I picked the fields from "Available attributes", however when I hit "Ok", I got a prompt :" Select all mandatory fields for inheritance so that child cases are created correctly".

I had checked my customised fields which I had already selected, but however I still get the prompt. 

Does anyone have any clues?

Thanks.