Admob ads don't load if initilized after Applovin

Question

In my "ad mediation" I first try to load Applovin ads and if it has no fill, then to load Admob:

//AppLovinAdLoadListener
public void failedToReceiveAd(int arg0)
{
    loadAdmob();
}

But for some reason, Admob ads don't load and neither callback onAdLoaded() nor onAdFailedToLoad(int errorCode) of Admob is fired.

Has anybody experienced that?