Changes to how ads are displayed recently came into effect. Google decided to restrict the period of of time ads could remain on an even-rotation setting. After 30 Days ads within ad groups would be optimised, i.e. the better performing ads would automatically be displayed more often.
However, this caused uproar among advertisers who wanted to retain more control over their ads.
Read the full article on our website: Changes to the Google Adwords Even-Rotation Setting.