If you are looking to see any Past Shifts when looking at your Shifts page, you must make sure that you switch the date-range setting drop-down list to include them. By default, when you navigate the Shifts page, the schedule opens to Today & Upcoming Six Months, which shows only the shifts happening within the next six months (within about 182 days from today). This means that any of your old shifts are hidden from the interface but are still there and need to be exposed using the date-range drop-down list.
To review past shifts, switch the date-range drop-down list to Past Dates.