The main reason you aren't receiving a reward is that you have too little RAM.
There are several ways to view the ads on devices with limited RAM:
- Disable the Google App Store. When the ad launches the App Store, it uses additional memory.
- Disable or uninstall all browsers. If the ad launches a browser, it uses additional memory.
- If you have a region where little has been built, try watching the ad there. A small region uses less memory.
- Clear the RAM in the system settings before restarting the game, and then watch the ad.
- If an ad plays successfully but the game resumes without a reward, it helps to exit the game properly (via the exit dialog!) and then restart it. After that, I usually received the reward.
All of these options are just workarounds when RAM is too low. EA is also responsible for ensuring that ads run correctly and deliver rewards even with limited RAM, after all, ads have been part of the in-game tasks for some time now.
The only requirement for SimCity is Android version 5. But back then, devices still had very little RAM. Players with large cities or regions use more RAM, and the features that EA has recently added require more and more memory.