maybe you should work on improving the ad placement. Just for sharing, I work on a website that gets around 2500 unique visitors per day (its a travel website) but gets around $10 every day from adsense. So i guess with 3000 and 4000 visitors everyday, $15 a day is near to what you should expect. However, if you highlight adsense all over your site, it will improve the earnings, but will make it look like an MFA site !!
That doesn't have to be be true. Some of the best adsense earners are ads that are well blended into the site. It's all about positioning.