Problem with unity ads after publish the game in play store

The problem is when the ads in test mode , it is working fine but when i publish it
it works fine (the ad show every 4 time when the player die):
the problem is : it show me (one ad or two just) and then nothing
the ad don't show up unless i quit from the game and open it again
i really want to know why ad just show one or two then don't?
i use unity 2018.2.2.1f

Sorry, English is not my native language