I selected the option to "update Shopify profiles with Klaviyo data." Why haven't all of my Klaviyo contacts synced over to Shopify?
For existing profiles in Klaviyo, we will start syncing those fields to Shopify the next time Shopify syncs any data about them to us. This is because, for existing profiles, we need their Shopify ID in order to update their information in Shopify. Profiles that don't exist in Shopify but do exist in Klaviyo will not be synced back as a net-new profile. They will still need to perform an activity in your Shopify store that add them to your Shopify account before Klaviyo will start syncing data back.