I import 600+ invoices a month. My customers have custom fields with information that needs to be shown on the invoice. This information does not fill on import

When you create individual one off invoices it does bring across the custom field details. I have tried both using the sample spreadsheet that qb provide with a couple of invoices and that doesn't bring the information across. I do use a third party tool for my import because of the number of invoices and that doesn't bring the information across either.  The only way I can populate those fields on the imported invoices is to go into every invoice change the customer to a dummy one I've created then click back onto the actual customer. This is very time intensive and can take me two to three days to complete each month. I'm not a happy bunny as desktop version did it no problem.