I have seen the same thing occur in my apps. Fill rates have dropped from 90+ percent to below 50 percent in the last few weeks. I have sent an email to the Amazon contact for an explanation, but so far have not received a response. .
Performance can vary due to many factors such as app content, ad placements, and advertising campaigns that match your user base.[b] Please continue to send ad requests to the Amazon Mobile Ad Network and we will continue to serve high quality ads to your users when available. [/b]