Is this KPI a Base or Composite KPI? If it's a base KPI then change the mode to Append which will ensure you are capturing each value into the KPI database table and then the KPI aggregation logic will work on each value captured over your aggregation interval.
You can always look at the raw data via the KPI Query in "Raw" mode to see the difference in the Update versus Append job modes but in general for Base KPIs please use the Append mode. Documentation is also available here:
Key Performance Indicators - SAP Manufacturing Integration and Intelligence - SAP Library
Sam