The four reasons a page is missed
In order of how often each is the answer.
Nothing links to it. This is by far the most common, and it is worth treating as a finding rather than a bug. If no page on your site links to it, a crawler has no route there.
The navigation is built in the browser. If your menu is assembled by JavaScript rather than delivered as links in the HTML, a simple crawler sees very few routes. Search engines handle this better than a basic crawler but not perfectly.
It is behind something. A login, a form submission, or a paywall means the page is not publicly reachable, so it should not be in a public sitemap anyway.
The crawl hit its limit. Our generator stops at 40 pages, and larger sites need a sitemap that regenerates itself rather than a one-off crawl.
Orphan pages are the real finding
If a page exists, matters, and nothing links to it, you have learned something more useful than a sitemap.
An orphan page is your own site declaring that page unimportant. Adding it to a sitemap asks search engines to treat it as important while your internal linking says the opposite, and that contradiction usually shows up as poor indexing.
So the better fix is almost always a link, not a sitemap entry. Link to it from a relevant page, a hub, or a navigation menu.
And if you cannot find anywhere it deserves a link from, that is worth sitting with. A page nothing on your site wants to point at may not need to exist.
This site had that exact problem
Worth being concrete rather than lecturing, since we made the mistake at scale.
Until recently venbit.ai had over two hundred pages reachable only through the sitemap, with no internal links pointing at any of them. Seven whole content clusters had no index page at all, so the only route in was the sitemap.
Adding hub pages and real internal linking took the average page from about 19 internal links to roughly 90, and it mattered far more than the sitemap ever had.
So when a generator misses a page, resist the urge to add it manually. Ask why nothing links to it.
Common questions
Why did the generator miss a page I know exists?+
Most likely nothing on your site links to it, which makes it an orphan page. Crawlers discover pages by following links, so with no inbound link there is no route to it.
Should I just add the missing page manually?+
Usually not. That papers over the real problem. A page in your sitemap that nothing links to sends two contradictory signals, and the contradiction typically shows up as poor indexing. Add a link instead.
Why can it not see my navigation menu?+
If the menu is assembled by JavaScript rather than delivered as links in the HTML, a simple crawler sees very few routes. Search engines cope better than a basic crawler, but not perfectly.
What is an orphan page?+
A page that exists but has no internal links pointing at it. It is effectively your own site saying the page does not matter, which is why sitemap inclusion alone rarely rescues it.
Sources