Fabric - MoPub: No ads found for ad unit

I am currently developing an iOS app (could be Android or sth else, that's not the issue here) and have just included the MoPub kit through fabric in order to monetize the app through ads.

After implementing a full screen/interstitial ad with the test unit id, the interstitial shows perfectly. However, when using my personal ad unit ID, the ad doesn't show up. What I get from the console is the following:

MOPUB: Interstitial controller is loading ad with MoPub server URL: https://ads.mopub.com/m/ad?v=8&udid=ifa:XXXX&id=1234567890&nv=4.7.0&o=p&sc=2.0&z=+0100&mr=1&ct=2&av=1.5&cn=EE&iso=gb&mnc=30&mcc=234&dn=iPhone7%2C2&w=640&h=1136&bundle=com.company.appId
MOPUB: Interstitial ad view is fetching ad network type: clear
MOPUB: No ads found for ad unit: 1234567890

N.B: if I do the exact same process with the banner, no issues; for some reason I ignore, it only happens to me with the interstitial.

Does anyone know how to solve this issue?