Ad Rotation Setting
Did you just let Google ‘look after’ this one? Bad choice. Google’s default setting is to optimise for clicks. Its probably the best option if you are only running one ad :). But since you are reading this, you are probably testing more than one ad and therefore should move on from this default setting. You should always ‘evenly’ test your ads because it is wrong to think an Ad simply is a way to draw more clicks.
What you can do to fix it:
An ad starts the conversion process and different ads have different conversion rates so choosing the ad that simply generates more clicks means leaving conversions on table without you even knowing it! In June 2012, Google forced all advertisers to show ads evenly for 30 days (sometimes 90 days) for whatever reason I’m not sure… might have something to do with wanting more click$. Anyway, Google then backtracked in October 2012 giving advertisers and ad managers the option to rotate ads indefinitely. Google doesn’t usually backtrack and is evidence that ‘default settings’ need to be discarded in favour of a more measurable approach.