I'm not a marketer but a book critic often asked for advice on this topic. And when I've referred writers to marketing or PR people who might help, those experts have sometimes told me that the writers expect too much for too little money and have unrealistic expectations about the services they can get for what they can pay.

It's true that academic writers can be a tough sell, but some firms specialize in them. Alas, I don't have any names of firms I can recommend (because, again, I'm not a marketer). But they do exist and may be what you need.