Did you manage the result sources according to the correct process?
You could refer to this official document to check your steps:
Manage result sources
There is also an article about the troubleshooting of Search results aren't returned, you could also have a look:
SharePoint Online doesn't return search results
Whether you want to create a content source separately for external web site?
Please create a new Content Source and select the type “Web sites” to be crawled and enter the start address, then start a Full crawl.
More steps refer to the article: