Changing Source Data Of An Existing PivotTable

I'm trying to change the source data of an existing PivotTable so that it is linked to a raw data source in another workbook. The workbook it pulls the data from changes every month so I made this flexible code that is meant to pull the raw data every month:


Sub ImportNewSource()
Dim Filt As String
Dim FilterIndex As Integer
Dim Title As String
Dim FilePath As Variant
Dim ThisPivot As PivotTable
Dim FileName As String
Dim ShtNum As Integer
Dim LastRow As Long

The PivotTable field name is not valid. To create a PivotTable report, you must use data that is organized as a list with labeled columns. If you are changing the name of a PivotTable field, you must type a new name for the field.

When I do it manually, it works just fine. There aren't any changes to the PivotTable field or anything like that so I'm stumped. I've tested the range that has the source data and its selecting the proper range and when I do it manually it works fine.