If you are noticing a drop in your adsense earning, please check these points.

• Your content is no more relevant to Advertiser.
• Advertisers’ ROI (return on investment) is negative. That’s why good advertisers are excluding your website from the list of websites where the ads are supposed to be shown.
• You will get the revenue share of their remarketing campaign only.
• Another reason is your potential advertisers’ business is down.

Problem from your end:

• Check if you ever tried to change your ad’s location in recent days. Or you might have tried to change design of your website.
• Check Adsense Viewable Impressions: You may have placed your ad on header to get more earning but you must know that if a user scroll down immediately after landing on page, then that placement is no useful. Your in-article ads pay you more because a user spend time reading the article and the as is show for quite good time.
• Your search ranking is down.