I was having this exact issue. Go to Settings - General - Reset - Reset all settings. Your phone will shut down so restart it. After this step uninstalling and reinstalling the app presented the ATT prompt each and every time. Also make sure Settings - Privacy - Tracking - Allow apps to request to Track is turned on.

Go to Build Phases - Linked Binnary with Libraries. Search for AppTrackingTransparency.framework. Remove it.

I was having this exact issue. Go to Settings - General - Reset - Reset all settings. Your phone will shut down so restart it. After this step uninstalling and reinstalling the app presented the ATT prompt each and every time. Also make sure Settings - Privacy - Tracking - Allow apps to request to Track is turned on.