If there is a fall in your Adsense earnings, you need to troubleshoot to assess the reasons for the decline. Your first step should be to gauge whether your page views are reducing or there are other issues.
Some key factors that need introspection include click-through-rate (CTR), page earnings for every thousand impressions, page views, and cost-per-click (CPC). These metrics are viewable on your account in the Performance reports panel. There are various Google resources that provide understanding on the reasons for the drop and the ways of maximizing your returns.