We have a web application with several site collections in our SharePoint 2013 farm. When regular users search for site content using "Search this site" box, it usually returns data for the most sites. There are certain sites where it doesn't return any data, even if I search being a site collection administrator. But, if I try to use the farm account, it returns data. I know it's some kind of permission issue, but I didn't find anything special in those sites, site collections and lists permissions.
Anyone knows possible reasons why this happens? I will appreciate any help.