If a custom component loads another custom component using FXML, the new FXMLLoader created by the custom component will not point to the appropriate ClassLoader, preventing the loading of the second custom component.

Although this is a custom code issue at the root, the SceneBuilder should probably sets the static default FXML class loader to point to the project's URLClassLoader for the duration of the call to the project's FXMLLoader.load().

see https://forums.oracle.com/forums/thread.jspa?messageID=10548526#10548526