Each account gets access to a limited number of ads. Thus, give more preference to the seasonal event ones like 'happy hour 'ads and play them first, and then watch the other ads(on the map for energy and coins, or 10⚡)

Due to a limited number of ads, the pop-up message saying 'ads in preparation' will appear after maxing out.
Force closing the game and reentering to refresh the data may help.

❗ But keep in mind that the ads will need some time to recharge.

📌 The ads may be even more limited or not provide the rewards in some regions due to political issues (restricted by the device's platform: Apple, Amazon, or Google).