The Search Lost IS (Rank) report tells you how often your ads aren’t showing up due to low Ad Rank. (Remember that Ad Rank is the order in which your ad appears on a search engine results page. Your Ad Rank is determined by the bid price you are willing to pay for a keyword and your quality score.) If you have 40% Search Lost IS (Rank) then that means your ad is not showing up 40% of the time due to low Ad Rank. The counterpart of the Search Lost IS (Rank) is the Search Lost IS (Budget), which shows the other reason why your ads might not be showing up.