@gouci there are some good suggestions here from some of the others. Because of your CTR, I would also like to ask where the banner is within the app and how do you see the user interacting with it? What kind of app is it (i.e. content); does it make sense for the type of users your app attracts to be clicking on ads?
Which countries are the majority of your requests coming from? The reason I ask this is because the demand from the advertisers in the countries you’re receiving the majority of the requests from could be low, which means the campaigns can bring down their bids and still win your traffic.