Why didn’t the Dropbox Folder’s Turnitin Similarity Report find the questionable materials in the student’s paper?

How Turnitin works is the company has agreements with a lot of journals, publications, websites, etc. that allow them to look through the websites' materials and check student submissions against it. They aren't able to do this with every website out there, so Turnitin has to rely on those websites to allow their contents to be scanned by the Turnitin website crawlers or robots. Some websites only allow big companies like Google or Yahoo to search their websites. Because of this, Turnitin might not be able to return any results from those websites. Unfortunately, that means that when you see suspicious quotes or paragraph text in a paper but Turnitin it didn't catch it, you still might need to do a search for the information because the website that the existing article or paper are from may not allow Turnitin to search their website for information.