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?
1. Write more. The more titles you have, the more you'll sell. Even if you only managed to accrue 1 fan, a new title means 1 sale.
2. Advertise. I still use BookBub, BookSends, and EbookBooster. If anyone has other suggestions, list them in the comments.
3. Sales. Kindle Countdowns and free periods always seem to goose sales in a title/series.
4. Increase prices. Huh? But you just said to try sales! I know. Try both. Amazon's new KDP Pricing Support Beta has suggestions. Try following them and see what happens (when I switched to suggested sales prices, my profit went up).
5. Collaborate. It never hurts to cross-pollinate with other writers' fans.
6. Try a new genre. The road to success is littered with folks who shifted gears, tried something different, and finally succeeded.
7. Change covers and book descriptions. Can't hurt. Or maybe it can. But you won't know until you try.
8. Try other retail outlets. If you're KDP Select, opt out and try other platforms. If you're on other platforms, try KDP Select.
9. More formats. Are you in audio yet? Do you have paper versions? Foreign languages? I'm currently #47 in Germany.
10. Billboards. I was talking about these back in 2008. A billboard is anyplace that points people to your work. Besides the obvious social media sites (Twitter, Facebook, Goodreads, Amazon), you should have a website that links to your books, allows readers to sign up for your newsletter, and announces what is coming soon.
11. Pre-orders. Amazon now allows this for KDP authors. I've used it to decent success (I was the very first self-pubbed author who had a pre-order page, back in 2010).
12. Pen names. Your name isn't selling? Invent a new one.
Now here are some things to avoid:
13. Don't whine in public. To quote Stephen Lynch, Charlie Brown will never get laid. No one likes a loser, and those who wallow in self-pity tend to bring more bad luck their way. Complain to your spouse over a beer, not online where you'll develop a bad rep. Always stay upbeat. And remember that the Internet is Permanent. Don't post shit that could come back to haunt you later.
14. Don't spam. Tweeting your ebook link twice a day is the limit, and don't do it more than two days in a row. People want information and entertainment, not sales pitches.
15. Don't quit. I've posted previously that if you can quit, you should. This is something I've been saying on this blog since 2005. (Damn, I'm old.) The fact is, no one knows when success (whatever your definition of success is) will come. I don't truly know if you'll ever get what you want out of this biz, but I do know that you won't get anything if you don't keep trying.
Like much in life, it all comes down to luck. Keep expectations low (no one owes you a living), keep working hard, keep learning, keep experimenting, keep at it, and you will improve your odds.