What is SEO?
It is three separate jobs that share one name, and a site is usually failing at one of them rather than all three.
The three jobs
The first is technical: can a search engine open your pages, read them and tell them apart. The second is what is written on them. The third happens on other people's sites, where being mentioned and linked to is what makes yours look worth trusting.
They are usually done by different people, in that order, and the third takes the longest by far.
Which one is yours missing
Search for a sentence copied straight off your homepage, inside quote marks. If your own site does not come back, the problem is the first job and nothing else matters until it is fixed.
If it does come back but only for your business name, the second job is where you are. If you appear for what you sell but sit below directories, it is the third.
What we do about it
We read the site against all three, write down what is wrong in the order it costs you money, and say which parts we would fix. Our search work covers the detail, and how long it takes is a separate and more honest question.
Questions worth asking
Is SEO a one-off job or something ongoing?
The technical half is largely one-off: fix it once and it stays fixed unless the site is rebuilt. The half that happens on other people's sites never finishes, because your competitors are still working too.
Can I do any of it myself?
Yes, and the most valuable part is the part only you can do: asking pleased customers for reviews, and writing down what you actually know about your trade. Neither needs an agency.
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