Hierarchical field not showing in page attributes box

Hi,
I set up a custom post type which should have hierachical pages.
In “Advanced Options” I checked the “Hierarchical” ceckbox and the “Supports”->”Page Attributes” checkbox.
Nevertheless parents selection field is not shown in the Page Attributes metabox on the edit page, there is only the box to set the order of the page.
What to do to show the parents selection field?
Thanks for helping!