Workers will only be visible on your roster if they have active shifts assigned during the time period you're viewing.
In the Roster screen, you can view the roster for Daily, Weekly, 4-Weekly, or 6-Weekly periods. To see all workers, you can either extend the timeframe (e.g., 4 or 6 weeks) or use the 'Roster Periods' tab, where all workers assigned to the roster period will be displayed.