To generate a map preview, a dataset can have multiple resources but each resource can only include one layer within it. Resources with multiple layers (e.g., multiple shapefiles in a single zip file) are not supported. In this case, the system will only create a preview of the first layer in the resource, however all the layers will still be available in the downloaded file. If you would like all of the layers to display, you need to create a separate resource for each layer.