Facebook Product Status - "products need one of the following universal identifiers to run in ads...

I am having a problem with some of my stores products not synching to the Facebook store when using the Facebook sales channel.

The Shopify & Facebook stores are all setup correctly & linking, but some of the items (not all) are giving the following error message:

“Products need one of the following universal identifiers to run in ads:- Global Trade Item Number (GTIN) – Manufacturer part number (MPN) – Brand name”

How can I fix this?
The products were all imported using Oberlo or more recently DRsers, so all the product information should have been synched when importing the products.
I have a LOT of product in my store, all imported the same way, but only a small handful will not synch up with the Facebook store.

Any help you can offer to solve this issue is appreciated.