You mean your conversion is dropping? Are you switching the ads up, how big is the area you're targeting, how long has the campaign been running? Could be getting stale, or the opposite, if you have new copy it might not be doing well. If traffic is up and ad revenue is down, you're just not getting the clicks, not converting, and need to do some testing to understand why. Although I don't know that a bad week is a long enough time to start worrying about it unless it's dramatic.
1- There would be a decrease in the traffic on to your website which has caused lesser exposure to your ads and less clicks.
2- The traffic you are getting mostly is not from U.S. Google's payout per click always varies according to the location. If the clicks are from U.S and Canada, its rate is very high as compare to clicks you get from Asia.
There are many things that can affect your clicks. Have you added more information to the page therefore making the AdSense box more hidden? Have you moved its position? Have you change the type of traffic you are getting to your site? All traffic is not created equal. Review any changes that your site has made from when it was converting good to now. If you haven't made any changes besides adding more traffic and if the traffic sources still the same then perhaps your content is stale.