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1) Which keywords are causing the highest bounce rates - perhaps it's the less relevant keywords with the highest bounce rate and/or lowest conversions that need are skewing overall bounce rate.
2) Review your ad text -- is it too general?
3) How relevant are your landing pages to keyword and ad text? Are your ads promising something that the landing page doesn't deliver?
4) Which landing pages have the highest bounce rate? How do they differ from those with lower bounce rate?