AdMob ads are slow to load

Good morning, I launched my application to the Playstore and I have a problem with the ads. It turns out that when a user installs it for the first time, the ads take around 2 hours to load, but when they have already installed it, they load immediately. This is a problem for me since I want to invest in advertising but I would lose all those first round ads. My question is, will this always be like this? It is my mistake? is it temporary? Beforehand thank you very much. I don't have much experience with ads