You can do it yourself. If you've ever used 'Black Hat' SEO methods then that is your problem. If you haven't it will more than likely be a lack of quality content or backlinks which Google doesn't see as relevant - low quality, not in your niche etc
Diversify your backlinks sources and your backlink anchor text. Might take a while, but it will work. SEO was never meant to be a quick jump to the top. Michael