Importing Data Linking Two External Tables

I have setup 3 tables.

1. Product data

2. Universal Part Numbers

3. Parts


Every Part has a parent product. Some parts have a universal part number.


I then import the parts data, I am linking one filed that contains the Product ref (SKU) and another that contains the universal part number (UPN), where there is one.


The import works correctly for the universal part numbers, with each UPN record being linked to the correct record in the Parts table. The link to the Product table is not formed however, and I can't work out why. Does anybody have any suggestions as to what could be going wrong?





More images showing the link and the result:


I have resolved this. There were some none unique (duplicated) SKU records. Once deleted the import worked as expected.