I'm in the same boat and came here hoping to find definitive answers. I've had a couple of sales where sales tax seemed to calculate just fine and then another one that didn't add sales tax at all. I have TaxJar connected. The product was not exempt and the billing address was within my own state so I know we collect sales tax. I cannot figure out why it didn't collect sales tax and I'm not going to go back and ask my customer to pay more now but I can't continue to lose money on sales because I'm having to cover the taxes. Hopefully someone will have the answer soon.
-Jessica