You have connected your master billing account from CloudFix. But, all the sub-billing accounts from AWS are not showing up in CloudFix. You are expecting more sub-billing accounts to show up in CloudFix.
CloudFix only shows the accounts where customers have purchased reserved instances or savings plans. So, this is expected behavior. There is no need to add any billing sub-accounts manually.
Note: This is applicable only for Billing Accounts. For Resource Accounts, each one is independent and will have to be onboarded if you want recommendations on it.