If you have the refresh rate set at less than 60 seconds, iAds will not refresh another ad no matter how many requests are sent. We suggest you set the refresh rate to 65 seconds.
NOTE: For ad units served by network line items (not Marketplace) except iAd, ensure that refresh rate settings on the ad network side are turned off. Refresh rates for MoPub should take precedence. This will prevent a double refresh rates which can have an adverse effect on performance.