A site collection (Its a Search Center site collection. We had some lists and list items under the site collection) and its contents are not listed in search results; though I have added site url as a content source in search service application.
Go to Site setting and Click on Search and offline availability.
Then select the radio button under “Allow this site to appear in search results?”
By default “Yes” option will be selected if we create a site collection with a template other than the “Basic Search Center”.