On our Intranet landing page search is configured and it is working, but under Site Collections that have their own content database, the search returns no results.
How can I configure search to work under those Site Collections.
Check the Search Account used for content crawling. The account needs to have read access to the site collections and its databases. To do so open your Central Administration => Application Management => Manage Web Applications => Select Web Application of Site Collection which is not crawled correctly => User Policy => If Content Crawl Account is not listed here, add it.
If this does not solve your problem, check the Crawl Logs in Central Administration => Application Management => Manage Service Applications => Your Search Service Application => Crawl Log. Check the error messages.