Amazon offers various metrics and services to authors: a glimpse into their own Bookscan numbers, a quick look at sales ranks and new reviews, a way to update their bio. It also offers something they call ‘Author Rank’, which is sort of like ‘sales rank’ but takes into account all your books (or so I assume).
This is a snapshot of my sales rank since the beginning of the service, which was about 8 months after Matchbox Girls came out. Book sales rank charts look fairly similar, with the same kind of downward curves and spikes. Each spike represents a sale (or maybe more than one sale, on good days.) As more books get uploaded into the system, existing books have to work harder to get the same spikes.
The Senyaza books reverted to me April 1; I uploaded Matchbox Girls at the $2.99 price point and the others at $4.99.
Citadel of the Sky was released May 15, which is also when I started actively advertising Matchbox Girls, with periodic $.99 sales.