Four failures cover most of the sites we are asked to look at.
The first is that nothing on the site says what the business actually sells, in the words a customer would use. There is a services page listing six categories, and no page about any of the six. Somebody typing the name of the exact job they need has nothing to land on, and the business never appears for the searches that would have converted.
The second is that the site cannot be updated. The person who built it is unreachable, the login is lost, or the platform it sits on has moved on. So, the hours are wrong, the services list is two years old, and a business that changes anything about itself has no way to say so.
The third is that it falls apart on a phone. Not badly enough to be obvious on a desktop, but text runs off the edge, a menu does not open, a form cannot be completed with a thumb. Most local traffic is on a phone, and most owners look at their own site on a laptop.
The fourth only happens once: a rebuild that dropped the old page addresses. Every link anyone had shared, and every result already stored by a search engine, now points at nothing. The enquiries stop within about a week and nothing visible explains why.