When your clients are small businesses, you will find they have a wide range of business and financial and legal acumen, and some will lack basic professionalism in most areas. That's especially common in the case of small family businesses.
I used to work at a company that targeted both small businesses and enterprise companies with different versions of the same product. Dealing with enterprises was predictable: they know about corporate governance, they have legal and accounting teams, etc. Every small business was a wild card.
If your clients have not been paying taxes so far, they might be afraid of owing back taxes or fines if they start engaging with the tax system and their past behavior is discovered. Also, some of them are probably avoiding taxes on purpose, maybe because they are crooks, or maybe because they simply cannot afford to pay. You probably aren't going to convince them to start paying taxes just to use your product unless it is incredibly compelling for them.