Have you ever measure the opportunity a keyword gives you. Off course you did, but only for short phrase keywords. These are most popular, competent keywords in your niche. It is hard to catch the target of these keywords against your competitors. If you are new in line then this would be impossible for you. Do you know that google get every day 20% unique search quries? Every 5th query is totally unique and never searches before. This means visitors change their searching strategies, keywords every day almost. This means to rely on specific keywords is not worthy any more. Try new experience with long phrase keywords.