So I am somewhat confused by the sales ranking report given by Amazon Author Central. I have published five books at this point (3 in one series 2 in another) and out of those 5 only 1 has a Best Seller rank. It is the 2nd book in my most recent series published a few weeks ago. The rank is somewhere in the 400k range but I imagine it falls every day.

The first two books I published had several sales but back in January when I started this I didn't think to upload them to KU. They achieved at least 3 sales each, but show no ranking months later. Is there some threshold that has to be met? I don't fully understand why only one of my books is whoing a ranking, or how to gauge whether its particularly "good".