The reason why you're seeing $0 for Tax Amount even though there was tax charged is because you are looking at data for a manually-entered transaction. This means you entered the payment as an external transaction rather than processing through your payment gateway (processor). These type of transactions will only show the flat amount charged in the export, and will not expose the tax charged for each line item.
If you want to see the tax amount charged for each line item in the invoice, please download the Adjustments export instead. Here is our documentation for that export: https://docs.recurly.com/docs/adjustments-exports