Fields when drilling down into a PivotTable

Hi RossBateman96 
Please try the below steps :

1. Add Posting Date to the PivotTable: 

    Go to your PivotTable and check the PivotTable Fields pane. If Posting Date is -     available, drag it to the Rows or Values shelf to include it in the PivotTable. 

1. Adjust Drill-Down Field Settings: 

              Right-click on the numerical value you want to drill down into. Select Value Field        Settings and review how the data is being summarized. Some configurations can influence which fields appear during a drill-down. 

1. Ensure Posting Date is a Column, not a Measure: 

Posting Date should be a column in your dataset, not a measure. If it’s a measure, it may not appear by default in drilldowns. If necessary, create a calculated column in Power BI to convert it into a field that can be included. 

If this works , Please let me know.