Tax code is null for migrated AR invoice - Very urgent

Hello Friends

We have a strange situation for only one operating unit during migration of data from legacy to Oracle

We migrate invoices from legacy system and the process is as follows

Extract the data from legacy system to a wordpad or note pad file

Then using our custom process it loads the data into staging tables

We use API then to load this data into oracle base tables

The issue is now with the tax code being null in oracle base tables, there is a value for tax code in staging tables however when we migrate it is not picking the tax code. i checked the lenght and configuration and looks fine. I am able to create an invoice manually using that tax code and it is working fine however when we migrate it is not pulling the tax code.