WPALLImport custom field settings

We need to import our products into WooCommerce and Atum Inventory. We are using WPAllImport to import the products but we are struggling to link the suppler data for Atum Inventory. We have tried to map custom field imports for this using _supplier_id _purchase_price and _supplier_sku which appear to be the field names required which we need to map. However when we run the import nothing is imported into ATUM at all for these products. Are we mapping these fields incorrectly? Is there a different set of field names we need to use? Any assistance here would be apperciated.