It happens to everyone.
Sales slump. Riding high in April, shot down in May.
No one knows why some titles sell and others don't, or why some sell for a while and then taper off. My own experience defies any long tail explanations. Sales tend to fluctuate, and I'll see spikes on titles I released back in 2009. Some of my ebooks have broken the Top 100 many times over many years, proof that there is no such thing as "backlist" when ebooks are forever. Your title is brand new to someone who has never seen it before.
A decline in sales is natural. So natural that I wrote about it back in 2010.
But what can a writer do when sales begin to flounder, other than panic, get depressed, and curse the universe?