Things to remember while creating custom content types through feature.
Sometimes you may face an issue that, once you create new content type it will not display it in the Site Content Types in the site collection. You can check these issues by analyzing the custom Ctypes.xml deeply. Make sure that the you are adding the content type IDs correctly.
Please check the below image for getting an idea about the format of the content type IDs.
If you create a new content type then the new content type GUID must need to add as a prefix to the inheriting content type ID by separating it by “00”
