Are there task dependent activities under the WBS that do not have resources assigned? See the following Oracle support knowledge article about how performance percent complete is calculated in P6.
Oracle Support Document 897937.1 (Why is performance % complete zero at the WBS summary level) can be found at: https://support.oracle.com/epmos/faces/DocumentDisplay?id=897937.1
This is not a software problem. It is the way P6 is designed. You must have resources assigned to activities to see the performance percent complete in the WBS grouping band.
Most likely this is because that there is no cost assigned for the activities. Performance % complete at WBS level is calculated as (EV (cumulative on the WBS) / BAC (cumulative on the WBS)). So, in case o no cost assignments, there will be no roll up value for the Performance %.