First you need to check your paper settings and see what you are excluding and searching against. If that all looks fine, you should contact Turnitin support with the paper info and site - a lot of things are indexed but not everything online. They can check if the site you found isn't part of their indexed materials.
Make sure you discuss your concerns with your institution. We re-evaluate turnitin each year against other options like safeassign to make sure we are still happy with the product. Ultimately though it's just a tool that helps provide info to instructors to make the final judgement - it's not designed to give that judgement.Reply