Interstitial ad not working in ios

Interstitial ad are working fine in android but in ios none of the ads are working, tried with testids as well. All I am getting is  this 
[CDVAdMobAdsAdListener interstitial:didFailToReceiveAdWithError:]: Failed to receive ad with error: (null).

and in onAdFailedToLoad event, sometimes i get unknown_error.

Is there any fix for this?