Before Google can rank your content, it needs to discover it, be permitted and able to evaluate it, and index it. If any of these processes go wrong, you might find that your pages don’t show up in the search results.
Most of the time, you can rely on Google to correctly index your content, all by itself. After all, this process is one of the foundational parts of what Google is, and does.
Simply putting your content online isn’t always enough, however.
If you have technical issues, low-quality content, or incorrect indexing controls, you might trip up during those processes of discovery, evaluation and indexing might go wrong.
Source : Yoast