turnitin pick only online resources. your institution repository is connect on web turnitin will picked your repository resources. if you not connect your repository on web turnitin will not check your repository.
Did you google the similarities? Were you successful? Is your IR open for indexing? Are the questioned IR-texts not yet indexed by Google? Are the similar texts in your IR really texts or maybe images (it is possible to scan texts as images - so maybe they shouldn`t be found as texts because they are plagiarized or perhaps they are not born digital documents and scanned as images)?
It is really interesting to know that plagiarized work can not be consider by turnit in. But turnitin should detect the similarities from the original work directly and this did not happen. Also notice that the documents are born digital.