Before the due date, any similarity to your classmates' work will not be shown in the Similarity Report.
You may notice an increase in similarity after the due date as your assessment will then be compared against the Turnitin database AND your classmates' submissions.
You may also notice an increase in similarity after the due date if an assessment template has been used. In a situation where students have used template text such as table headers or report sections, these sections of text will only show as matches within the class group after the due date has passed, and will likely increase similarity percentages for the whole class group.
Likewise, if two or more students have included the same piece of text, Turnitin will display the existing external database matches prior to the due date. Once the due date passes, these student papers will also match against each other.