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.