amazon sales flatlined after a decent first six months

First split it into traffic or conversion, because the fixes are completely different. Business Reports shows sessions and unit session percentage per ASIN, compare the last month to your good months. If sessions dropped: check your organic rank on your main keywords, whether you went out of stock at any point (that knocks ranking down and it's slow to come back), and whether you cut ad spend. New listings also often get an early lift that fades. If sessions are fine but conversion dropped: look at who's sitting next to you now. New competitors at a lower price, a couple of fresh bad reviews, or losing the Buy Box will all do it. Which one does yours look like?