For about a year and a half I had a good run with basic AMS Manual Keywords ads — those ad farms of thousands of keywords. Then, in the fall of 2018, something significant changed at Amazon, besides just the AMS reporting system upgrade, and my ads which had been running at a smallish but steady profit basically stopped working well.
At the time, I didn't concentrate on the issue, content to spend several months working on getting my feet wet with Facebook Advertising while my AMS ads ran feebly in the background (at least I wasn't losing money on them). Let's just say that two separate attempts at Facebook product ads ended unsuccessfully and I shut them down to rethink my approach.
So, there I was in June with no effective advertising running and, let me tell you, that is no place you want to be. In the absence of any recent new release, my organic sales on my primary (1st-in-series) books are, at best, less than 1 per day.
But there's an upside to poor organic sales numbers — they make a great background to test against for advertising!