You can search engine optimize your website or blog all you want with keyword rich copy but if the information you are providing is stale, repetitive, robotic or plagiarized then you are not likely to attract many visitors. Furthermore most search engine spiders and robots will penalize you for plagiarizing and rewriting content from elsewhere by not including your pages in its search engines.
Another reason that your SEO content could be failing is not necessarily that you have committed the sin of plagiarism or being out of touch, but you may have chosen to search engine optimize copy for a product that has a glutted market. If a lot of people are writing about the product then it means you have failed to properly define your niche market in terms of how it will make a profit. Trying to make a profit in a glutted market is often useless and it is also a bane to SEO marketing as the keywords that are used to market glutted products are often too general and too popular to bring you the targeted customers that you need.
Sometimes the problem is that you have identified yourself as some kind of online consultant or guru but then somehow did not live up to the expectation of the people who have visited your site. A simple mistake in your SEO, niche language or keywords (or even a simple spelling mistake!) could have many people turned right off of your site. You might have also hired a ghostwriter who knows nothing about their assigned topic and you have no idea that your book is plagiarized from sources all over the web. Who wants to pay for information that they can already get on the World Wide Web for free?
Always remember that SEO selling always has to do with your credibility. Trying to sell things with a web page full of innocuous advice or copy full of nonsensical keywords never works.