I am similarly baffled that about a month ago Amazon clearly stopped showing my ads. Sales have dried up and I have been too busy to deal with what on earth to do next. For four years, my first-in-series ad has reliably brought me sales and reads. Now, nothing. I boosted the daily budget very substantially, but nothing happened. I re-enabled a bunch of my Amazon ads for the other books in that series, including a series ad, but again, nothing.
I obviously have to pause that ad and write a new one and that's something I have not been looking forward to. Plus, a new ad might not work, so then I'll have to try a Facebook ad, hoping to bump sales enough so Amazon will again show my ad. Arggh.