The issue can be complex. Are you seeing test ads on your device? If you are, the Ads are shown on your own device. But maybe the users device isnt able to show it. Due to the issue with being it remaining hidden. Are you seeing requests? Are they interstitials?
If they are banner, have you checked that the devices of users who downloaded your app are able to show banners in your app?
AdMob doesn't require any number of minimum downloads for displaying Ads. It can be shown even with 1 download. Just the issue maybe is due to your implementation.