First you need to identify if it an issue with the data or, the pivot table itself. If you double click the empty field, does a new worksheet open showing data that corresponds to that cell?
If you select different fields for the rows or columns in your pivot table, does you empty field “move” or does it stay in the same place? If it stays in the same place, obviously its the format/properties of the cell in excel preventing data from being shown.
If it “moves” then you have an issue with the data itself, so check your data source for any oddities.