On Setup > Accounting > Tax Setup, Enable Tax Lookup on Sales Transaction = T
Default Tax Code = (a default tax code is set other than -Not Taxable-)
On the customer record, Taxable = T
On the customer record > Address tab > Default Shipping Zip code is present
Tax Groups include the customer's zip code
Not- Taxable tax code populates on the taxable sales transactions of customers with valid shipping addresses
Make sure that the nexus is on the Nexus tab of the subsidiary record (Setup > Company > Subsidiary > Nexus). If the nexus is not on the subsidiary record, even if the zip code is on the tax group the tax code will still default to -Not Taxable-.