How to get Amazon and Shopify product linking working?

I have a lot of inventory and upload via csv. I have the product auto-linking by SKU enabled in the Amazon sales channel. It is giving me the message “No matching SKUs were found between your Shopify and Amazon products” in the sales channel settings, even though I can manually verify that matching SKUs are present in my inventory. When clicking “go add Amazon sku” in the channel settings, it takes me to the “bulk editor” where every product has an SKU listed but the Amazon SKU field is blank for every product.