Possibly a content issue with your site. What are the other competitors' PR? If you're not in the top 100, google obviously doesn't view your site as relevant for that keyword. Keep in mind it's good to vary your anchor text and not get all incoming links with exact anchor text. It looks unnatural. Also most sites get 80%+ off their traffic from long tail keywords so while it's good to go after your target keyword, you may find you get more traffic from other less competitive keywords along the way.