First you need to identify the reason of Search Lost Impression Share. It is basically of two reasons.
--- Search Lost IS (rank):- The percentage of time that your ads weren't shown on the Search Network due to poor Ad Rank in the auction.
Then you can increase the Bids and Improve the QS so that you can improve Search Impression Share.
----Search Lost IS (budget) : The percentage of time that your ads weren't shown on the Search Network due to insufficient budget. In This case increasing the bids will not help thus, you need to increase the budget to gain the higher Impression share.
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