This is just my $0.02 from self-publishing for the last year, so take it with a grain of salt, a shot of tequila and a lime.
1) Don't ever compare yourself to another writer! Otherwise, I would have slit your throat, run your body through my Cuisenart and buried the pieces in Friend X's compost pile months ago.
2) Genre does matter. All those genres that agents and editors are saying are dead, like romcom, vampires and thrillers? Those are the ones kicking ass on Kindle right now.
3) Sometimes, the social media circus doesn't mean jack. The books I write under my name and plug the hell out of? They're doing 'Meh' sales. My alter-ego who's done zero promo work for her genre? She's kicking ass on both the Amazon and B&N charts.
4) Put more books up. Hubby did a rough calculation back in 2010. A writer needs an average of ten books to hit the tipping point of self-sustaining sales. Some do it with one book; some do it with the twentieth. I didn't see much of a change in sales until my seventh book went on sale. I noticed Friend Y stated kicking butt around her fifth.
5) Did I mention write more books? Try a different genre. Take a chance on something different. If you're worried, publish under a pseudonym. Don't stick all your eggs in one basket.
In other words, don't sweat the little stuff and keep writing.
I'll get off my soapbox now. Got to write the black moment scene, and I'm procrastinating.
Now mind you, my friend thought her sales sucked when in fact, she's doing quite well. Success is a relative term in this business. Only you can define what it means for you.