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It's somewhat hard to diagnose without a URL. If you want to use your private Q&A question credit for the month, you can ask this question and only SEOmoz staff and associates will be able to view the question and URL. General advice would be to look at Open Site Explorer to see the rest of your backlink profile to make sure that you don't have spam pointing at you, look in Google Webmaster Tools to see if there are any messages from Google about your site being hacked or violating guidelines, and looking at the other sites ranking for your keywords to see if they are ranking because they truly do have better content and links than you.