Make sure that the part is manufactured, has a cost template for manufactured part that includes labor cost element(s).

Check that the routing is in status Buildable

Check the Labor Class on the routing operation and make sure that there is at least one factor defined for either the labor set up or labor run time and a crew size of at least 1

Check the math on the factor as well, especially if the factor is based on units per hour rather than hours per unit. Sometimes the amount calculated is a fraction that is less than a penny per finished unit and therefore would not show an amount. Standard Lot sizes may also be useful but not necessarily needed.