Shopify requires that "inventory tracking" is enabled for a variant's inventory quantity to be updated.

To enable inventory tracking, simply go to the variant detail page in the Shopify Admin and make sure the "Track quantity" checkbox is checked as shown below:

Once this is set, EZ Inventory will be able to update the variant's inventory levels.

More info here: https://help.shopify.com/en/manual/products/inventory/track_inventory