Also if you do not have Google Search bars they are good to add on an article website and like everyone else said change the positions of your ads aswell.
Try changing color, that worked for me on one of my niche sites althought a lot of people are against it. I used a color that actually stood out from my background instead of blending in as many people teach
Alexa rank 40,000 is huge. Install google analytics code if you have not already and see how many unique views you get per day and what are the search queries and bounce rate. You really supposed to make tons of money with adsense with the site that popular! Edit: you can't get Alexa rank 40,000 with "668 Impressions Daily". I only can conclude some pages that receive traffic do not have Adsense on it or do not want to load it (for different reasons e.g. no ad inventory for keyword/geo combination)
If you are getting visitors but no ad clicks then definitely thr is some problem with your ads placement. Try to place the ads in between the content and match the colors as well.
the problem is that his ads do not show. Alexa rank 40,000 corresponds probably to 20,000 daily pageviews at least.
Give you ads a new placement .for eg if you running a blog then try to put them before the post .Give them more visibility & between the content .