Ads are not stable on some devices.

I have implemented video ads in my android game. Most users watch ads perfectly.
But on some Android devices users say that ads are rarely seen or never seen.I I have tested my code on some devices(Xiaomi Red Mi Note 4 ). At first it was working quite normally. Then the ads were never ready. It was just waiting in while loop. When I waited a few hours and tried it again, ads appeared. Then the ads disappeared again.
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