When you get an invoice that does not print the tax, take a look at the entry and determine if the tax is being displayed there properly. On the 'Totals' (4th) screen of the invoice, you'll see the tax value at the left, and individual jurisdictions to the right. It may be that you'll see zeroes there.

Once an invoice is posted in Order Entry, it cannot be edited to make changes.
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If you simply want to print the Total Tax for the invoice on the last page footer, I would recommend using the F15 code (Total Tax). Individual tax jurisdictions can be printed using the codes F2 to F6. 'F' codes can be found in the 'Invoice Totals' area of the Spec code list. The Value-added tax (V) codes really belong in a detail section with a type of 'Value-added taxes'.

If you make the change to use F15 as the Total Tax code, you could attempt to re-print your invoice to see if that makes a difference. The easiest way to change your spec is to edit the field Properties of your current V5 tax code, and at the top of the window, use the drop-down field to select F15. This way you don't have to re-position or re-format the new field.