Tricky question Koha ... are your ads relevant to your content? Have you made any major changes on the site?
My earnings are off by about 20-25%. This started happening about a week ago. I've noticed some lulls like this in the past, let's hope this is one of them.
This happened to me once in the past where revs dropped and then spring back.... I am feeling a decrease aswell so hopefully it will even out, but it could be bigdaddy serps change moving in. (for me atleast)
Relevance is determined by Google & content. I have added some CJ ads but the CTR on them is very disappointing. They will probably go away. Adsense CTR is down only minimal and impressions or up it seems that the ads are not paying as much. Hope it's only temporary.