Adsense is one of the premier online advertisement companies from Google. It has wide inventory with relevant ads to display on websites of any niche or market. It is hassle free unlike other advertisement with reference to its ad code implementation, payment for your earnings etc.
But many make very few dollar or cents from ad-sense due to various reasons. In the recent past its cost per click (CPC) has gone down much like to few cents like 2 to 10 cents even for single click even if there is high click-through rate (CTR).
Hence some busy sites have banned using ad sense from their sites and have opted for direct advertisements. But this option is not possible for small publishers and have to rely on ad sense.
Sometimes you may notice sudden fall in earnings, clicks in ad sense. This can be because of
1. Change of ad units: When you change from one ad unit to another like large rectangle to leader board etc. The new changes take time for ad sense to establish which types of ads to display on your site after the changes.
2. Blocking ad types: Adsense team advice to block some category of ads to see if you can have relevant ads on your site. But when you do some ad blocking, the CPC, CTR goes down.
3. Changing ad unit placements: Changing the place where you display your ads. When you change the place of ad display on your site again earnings fall down.
4. Seeing your own site many times: Though this is not confirmed by many, it seems that seeing your own pages with Adsense ads many times in a day decreases ad relevancy and there by earnings and clicks.