CSV import - SKU and Stock not working


I am trying to import 400 or so products to a new site. I gave up using the MarketPress built-in importer and now use the WP Ultimate CSV importer - it is more advanced and asks for each field to be filled in specifically.

I am using the following:

Product Title - post_title [works]
Description - post_content [works]
SKU - mp_sku [not working]
Price - mp_price [works]
Sale Price - mp_sale_price [not used]
Tags - product_tags [working]
Categories - product_category [working]
Stock - mp_inventory [not working]
Weight - mp_shipping [not working]
Image - feature_image [working]

If anyone can see from this why some of the columns in the csv are not importing, and could let me know the solution - it would be really, really appreciated.

Thank you for taking the time to read this topic.