In CustomPress, what might cause the Featured Image block not to appear on the Edit Post screen for

Hello. I'm just setting up CustomPress on a site that has pre-existing custom type posts that were created manually in functions.php. I'm delighted to say that the plugin is recognizing the old posts appropriately. I wasn't sure it would work that well.

But my question: These posts have Featured Images, and I have set up the type properly in CustomPress, having checked the "thumbnail" box under "Supports". The Featured Image block is not appearing in the Edit screen for these posts, though. I have also checked under "Screen Options", and it is not available there as a checkbox.

On the other hand, on the template side, the right images are showing up.

What might be the reason for this?

Many thanks.