If you use 'Optimize for clicks' in your Ad Rotation settings, your ad testing can take significantly longer because Google may favor one ad over another (e.g., based on CTR data) and show it most of the time. Hence, it's recommended using 'Rotate indefinitely' for your campaigns. You'll receive an alert whenever a campaign is not set to this setting.

Click on the button to see the filtered campaigns. 

You can change this setting directly in Google Ads as described here: https://support.google.com/google-ads/answer/6034106?hl=en-GB