Web content writers may not actually know how to write for search engines. You need to be clear about the scope of work you expect from your web content writer.
Search engine optimisation is a major part of writing for the web since search engines are the ones ranking online content. Even if you have a great content, its chance of making it on the first page of search engines results is low if it has not been search engine optimised.
SEO is an online strategy that you must apply in your web content. If your web content writers don’t know anything about SEO, you can’t expect them to be as productive as you want them to be.