Rota related settings can be found by navigating to:
Administration -> Company -> Settings -> Time Tracking Settings.
Rotas have just one setting which is labelled "Rota Department Access".
If this is set to "Yes" it will show all employees who are in the Managers Department and therefore restricts them to only build Rotas for these specific Employees.
If the setting is set to No then Managers will see all available employees company wide.