Unfortunately this is the downside of being an IP rights holder - policing is a PITA. (Amazon, do their credit, does offer a brand registry, but that protects against trademark infringement, not copyright infringement.)
In your case, assuming they did not do exactly what they said they would do (remove the infringing content from the specific listing page referred to by the ASIN), I'd have your attorney send a takedown notice to the following address:
Copyright Agent
Amazon Legal Department
P.O. Box 81226
Seattle, WA 98108
If your content is on multiple product listings, you'll need to identify each ASIN.