OK. Firstly isAdReady==false will occur when there is no ad available to show. Ie it is a natural condition. You can mitigate it by using mediation.
But in your case it is likely that no ad was available to show because there had no been enough time to send the ad request and receive a response before asking that question.
What you need to do is to call loadAd early. Ie when you game first starts. Then at a natural break point in your game (after the player dies) check is isAdReady is true, and if so show the ad.
When you show the ad or when the game starts up again, call loadAd again so that you will have an ad ready again the next time you want to show it.