The market keeps changing, and some things are out of your control. Yeah, I know, duh. Still, it's true. The minute you think you have it all figured out, something changes. I sold huge quantities on B&N, then they realized that erotica was dominating the bestseller list and "corrected" what they saw as a problem. I did great with Select once; a few months later the luster was off the Select rose, and it no longer did much for my sales. I put up a short I thought might do really well, and Amazon slapped the "mature" filter on it, making it more or less unfindable. And so forth and so on. You can't control everything. All you can control is your own product.
Considering self-publishing? As with all writing, there are no guarantees.
Author Meg Harris shares her insights on 3 years of indie publishing.
As Meg says, keep in mind: "The market keeps changing..." That always happens. Be prepared to adjust. You're not doing anything "wrong". It's simply market forces.