I can only guess what might the problem be because Admob doesn't really tell the developer anything when an ad is not served, so it's really hard to figure out anything at that point.
Having said that, I don't know how often the ad requests are being made in your app, but Google Mobile Ads SDK (which takes care of the request under the hood) might just be ignoring them for iOS devices if it determines you are making too many.
Try checking your Admob account for any hints about "limited ad serving" or something similar, maybe your app has been flagged for some reason and that's why not all requests get back an ad.