Repeating sessions is a great way to strengthen your practice. If you scroll back in your timeline to repeat a particular session or pack, these will count towards your Run Streak but not total time meditated, nor total number of sessions completed. We are working on fixing this in a future update of Headspace, however.
If you would like to repeat a particular pack, you can queue it to your account through the 'Series' link from your timeline. As you progress through the pack for an additional time, your stats should then update to count these towards your Run Streak, Total Time Meditated, and Total Number of Sessions Completed.