If you see a newly imported listing is unlinked, check to see if the listing has a SKU number. Some channels, like eBay, don't require a SKU. As a result, sometimes sellers don't use SKU numbers on their listings.
Sellbrite does require a SKU to create a product, so if this happens you'll need to manually create a product from that listing.
To do this, edit the listing, choose "Create New Product", and add a SKU during the creation process. The listing will now be linked to your newly created product, and you can sync inventory to that listing (and any new listings you create from the product) by editing the inventory of your new product.