Same problem here.  We have a very simple accounting and bookkeeping system. We operate on a "cash" basis. We create invoices, tag the services as taxable or non-taxable.  At the end of the quarter, I run the sales tax liability report.  The gross total seems to always be correct as I match it to the Sales by Customer Detail report.  HOWEVER, the non-taxable column is off by $8,505.17 for the same period, same accounting method. I spot checked many invoices in the "detail" of the non-taxable column and they are ALL marked as taxable items. Where is the "non-taxable" column pulling it's information from?? I've called the support line and the same answer for every issue - "it's under investigation and we will email you when it's resolved".  OK, two years later, I'm still waiting for answers!  

Does anyone in this forum from QBO have an honest answer?  Just say, "I don't know why" if that's the case.  That's better than you telling me and all of us on the forum that have issues some lame example of how to work it when it doesn't apply to any of our issues.