Tax advice was not followed by the company

I am a tax advisor, and I am often asked to weigh in on a specific tax issue. 6 years ago I issued advisory related to a sales tax treatment of specific products that this company sells. The advisory was distributed to all people involved but was not followed. What I mean by that is that certain products that were supposed to be charged sales tax were not charged the tax. Because the company is now for sale, we are doing due diligence. We are fiding that the tax that was to be charged was not. And nobody communicated this to me.  When I produced the evidence that I sent advisory to parties in charge of setting up the sales tax correctly 6 years ago, I was told that I make people look bad because the email that was sent is over 6 years old. I apologized and said that I did not mean to call it as such. I did not, I resend it as a proof of communication. I always track my movements. But after that I am thinking to myself, should I apologize or am I owned the explanation why the directive was not followed. Please, help me understand.