Do you need a SEO agency?
Does your website generate qualified traffic?
Having a site to look pretty is useless. Its goal must be to generate qualified traffic and convert this traffic into a prospect. If your site is only visited by a few dozen people per month, it is unlikely to provide you with enough customers to grow your business.
Do you publish content but it does not appear in search engine results?
Unfortunately producing content is not enough. It is not uncommon to see websites with several hundred articles and barely a few dozen visitors per month. It is not so much the quality of the content as the absence of a clear and relevant SEO strategy that is in question.
Internal linking, netlinking, duplicate content, structured data, these terms mean nothing to you?
In reality it is not very serious and the jargon is never a sufficient element to justify the use of specialists. On the other hand, an agency specializing in natural referencing will be able to explain to you the issues behind these terms and sort out what is necessary to achieve your objectives and what is not.
“My web agency told me that my site was SEO optimized”
At the risk of making enemies, we hear that all the time. I don't think the agencies in question mean anything badly. But there is only one judge of the peace, does your site bring you customers, do you have significant traffic? Otherwise. Your site is not SEO optimized, period.