When an in-page responsive design is served onto a site, the design will fill as much of the ad slot it is served into as it can. Some examples of in-page responsive designs are Responsive Mastheads, Fixed Aspect Ratio Designs, and Pushdowns.
If an in-page responsive design is served into an ad slot that is not set up as a responsive ad slot, the design will still fill the ad slot. Since the ad slot remains the same size when the webpage is re-sized or displayed on a different resolution, the design served into the ad slot will remain the same, rather than responding.
To allow for an in-page responsive ad product to function in a responsive manner, it's important for the ad slot the design will be served into to also be responsive.
For more information on how to create a responsive ad slot for your site, check out the IAB Slot Definition Techniques article. Information on some key topics in the article are listed below: