Ads not mobile responsive

I’ve just recently noticed that the 728×90 ads are not displaying correctly in mobile view. I have it set up to display after 2nd paragraph on all my blogs. It displays fine on computers, But on phone, it doesnt reduce size to fit screen and so the beginning and ends of the ad are not visible.

How can I make the 728×90 ads mobile responsive – to show completely on phones?
(my 300×250 ads show just fine on all types of screens).

Thanks.

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