Thanks for your reply. This is strange – I used Product Feed PRO to edit the GTIN and brand fields when they were still included in the free version of your plugin. The shop was part of a multisite network then as it is now, and it worked fine.
The GTINs I added back then are actually still in the feed – I just can’t see the fields in WooCommerce to edit them / add them to new products.
Has something changed? Is it possible to try the Elite version before I buy it to test for myself?