The way the system is currently set up, previous submissions by the same student, in the same class, using the same account, will not be initially flagged within the new Similarity Report. The match will be found, but it is initially ignored. If you go into the second submission, as an instructor you can re-include the match to the previous paper. The report will then show the words that matched the previous submission.
In order to bring back the match,
Open up the new Similarity Report.
Click on the red "Excluded sources" icon to the right of the screen (looks like a circle with a line through it).
Click on the check-box next to the source you'd like to re-include and then select the "Restore (#)" button (or the “Restore All” button to restore all excluded sources).
Note: If the "No Repository" option was selected when the assignment was created, the paper is not indexed by the system and no match will be found. Additionally if the assignment was set without the "Student Paper Repository" search target checked, no matches will be found to any student papers (including the student's own paper).