You may face that altough:
- You have permissions to edit the layout
- The page Layouts are available to be changed
- The page is checked out
- And the page "seems" to be in edit mode
You still can't change the layout. This seems a bug, as the layout button is enable by the querystring EditMode. So try to add ControlMode=Edit parameter in the querystring, and page layout should be enabled if the prerequisites are met.