When a business tax record linking to a sole trade client is created, the system also creates a Has Trade Of association, which is required to ensure the link between CCH Accounts Production and CCH Personal Tax will work.
You must ensure you have this specific association type in place if you need to import accounts from another client record (i.e. a sole trade client).
If you find a case that isn't working and add an association of 'Has Trade Of' and select the sole trader client, the accounts import will then work successfully.