Admob Mediation not working


Can someone please tell me how did you manage to set up AdMob Mediation (for those using it, of course)?
We have implemented AdMob Mediation in some of our apps a few months ago, but we can’t make it work. It’s all good for AdMob Network, but when it comes to 3rd party networks, nothing comes through. They count some requests, but impressions 0 (hence, fill rate 0%, eCPM 0$). We are trying Millenial Media and MobFox and it is the same with all of them. And we have traffic from lots of good countries like Germany, Spain, USA, UK, Italy.
The process we went through with our apps is: create the app in AdMob, get the Mediation ID, put the SDK in the app and upload it to Google Play.
Are we missing something? Is there someone else who went through the same thing?