I'm beginning to believe that books have a natural "stickiness" - meaning that they stick at a certain selling rate without further promo (assuming the book has been launched well, reviewed well, and given a scattering of introductory promotion in the world).
That stickiness may be high on the charts, or in the depths of the Amazon dungeons... and it can temporarily be boosted by a promo like Bookbub, but generally will fall back to its natural "resting" spot. That spot can can be shifted by changing price or cover or description, but in the end, the book appeals to a certain number of people (large or small) and will stick at a certain level.
Example 1 (the high end):
Years after its release, Open Minds "sticks" somewhere in the top 1000 free books on Amazon, without me doing any promo. Periodically I will goose it with a Bookbub ad, but even between ads, it stays there.
Example 2 (the low end):
In July, I had a Bookbub ad for my middle grade novel Faery Swap. I'd worked hard prior to that to get it lots of reviews (26) and kept the price at $3.99 since launch last December. It sold in single digits per month. I knew that with only one MG book to my name it was going to be tough, but single digits is PRETTY TOUGH! Anyway, the Bookbub at 99cents moved 400+ copies, far more than Faery Swap had sold prior to that. BUT in the process, I discovered the book had "stickiness" at the 99cent price point - i.e. it sticks on the top 100 chart for its category without further promo by me. I told myself I would keep it at 99cents until it dropped off the top 100... a month later, it still hasn't dropped off.
Faery Swap is now selling double digits (on pace for nearly 50 books this month) and appears to have found its "sticky" point.
Those two books have different audiences and different appeal... and different sticky points. They're both written by the same author (ME!) and are both quality books (IMHO - they review well). Their "stickiness" levels are a reflection of the market.
One reason I encourage writers to try different genres is precisely to see how strong genre/market influences sales - it makes whether your books are selling or not a lot less personal. My goal (always) is to write books that (basically) sell themselves. Writing in different genres will show how that's easier to do for some books than others. Some books simply will sell more than others. Tis the nature of the beast.
My current plan is to leave Faery Swap at 99cents until it drops off the top 100 charts, then hike it up to $3.99, then do another Bookbub when 6 months is up (the time between Bookbubs)... and basically do no more promo beyond that.
That way I can concentrate on the most important thing: writing the next book.