In any endeavor, there is a vast pyramid ranging from the least successful to the most successful. By all measures, it is not a fair pyramid. By that, I mean that any two writers of equal skill and measure can have fantastically different outcomes to their writing careers simply because chance, personality, and serendipity play such a huge part in who becomes successful and who does not.
In traditional print, any single publisher can only handle a finite number of books per year. Who do they sign? Given a host of potential writers, they must choose. A few will win contracts, but most won't. Many will be sorted out due to merit, but many will also be sorted out simply because the pool of good writers is bigger than the pool for published writers.
Success works the same in the self-published world. I sell very few books. My works hover at 1 million on the Amazon best seller list. If I sell one book, I jump up to #100,000 on the best seller list. That is to say, one sale beats out 900,000 other competing titles.
I want you to consider that. One sale puts me in the top tiers of the pyramid. However, one sale does not make for a successful publication. There are many folks out there who get more sales and get them easier and get them harder. That's the vagaries of the marketplace. The pro writers chase the market, writing for what's hot. Some writers are good at picking up social media followers. Some writers are pros who've begun self-publishing, and so already have a fan base. Other writers are just unlucky, writing good books that don't have popular appeal. Some writers are terrible writers. Many simply get lost in the shuffle as there are always new books showing up.
For whatever reason, I sit here at the bottom of this pyramid scheming. How do I go from down here to up there? That's the question that all of us down here want answered, so we read and learn from success. The problem with this is that so many people are learning from success that any single successful vector will soon be flooded and so become useless.
To those who have found some measure of success, I honor you. I do not sit here in (too much) jealousy. The simple truth of the self-publishing game is that rewards are few and far between, and those with any success should count your good fortunes. You may be doing the right things, and work hard at those things, but even the right things fail more people than they reward.