If your followers installed the app you promoted but you didn't receive any coins, it might have happened for 4 different reasons:
Your followers haven't opened the app
In order to receive coins, your followers have to install the app and open it - and many of them forget this last part. Coins will only appear in your account after that.
Your followers are not in a valid country
Installs cannot be converted into coins if your followers are not in the targeted countries. For example, if you're promoting an app that is only valid for the US but someone in Japan downloads it, this installation won't be converted into coins.
Your followers don't have the valid operating system
For example, if you're promoting one app for iOS devices, you won't receive coins for the installs on Android.
Take your time, go make yourself a cup of coffee
Usually, coins appear some minutes after someone opens the app, but sometimes it can take up to 2 hours. It all depends on the app.