A bit of a novice here but if this helps someone else with their headache...
I was having the same problem but was unable to find the cause or solution here or in the wiki.
What I found after beating my head on the keyboard for a few hours, was that you may also need to check your settings in:
Sales ->> Sales Types
If the box marked " Tax included: " is checked, FrontAccounting seems to assume that the tax is already included in the item price. Therefore, tax will not show on the sales order or invoice.
It was hinted at by alanmc in one of his replies above but I completely missed it.