Players, especially attackers, could have a period of downtime sometime in their careers. During that period, they might not be scoring goals regardless of the hard work they put in. It is also important to note that Mbappe has spent a long time in the French League and might need some time to get used to the La Liga. France recently beat Belgium, but Mbappe played as a substitute; I don't know why the coach decided to keep him on the bench. I suspect that he wants to focus more on his task in Real Madrid because much is expected from him. Mbappe is a gifted player, and he is still young, he has the advantage of working under an experienced coach like Carlo Ancelotti. Give it some time; I guess he will improve.
There is no doubt that Mbappe is a talented player. Mbappe has the ability to score a lot of goals. We haven't seen Mbappe play well in the first three games of the season. However, Mbappe managed to score two goals in the fourth match. Madrid won 2-0 against Real Betis and Mbappe scored both goals. Mbappe is slowly adapting himself to the Madrid players.
But Ancelotti's main challenge at the moment is trying to keep his squad's performance stable in the next few matches. Because right now several players in Madrid's squad are facing injuries. And we haven't seen so many important players of Madrid facing injuries at the same time in the last few years. And finding a suitable replacement is now the biggest challenge for Ancelotti. As many players are injured, Ancelotti is unable to apply any new formation or match strategy. Madrid will need a long time to find strong form again. In the next few weeks we will probably see some erratic performances from Madrid.