I was able to join the provider of the music store where they send them to repair. The guy told me that it was maybe a way of getting used to blow of a certain way. He doesn’t think something is wrong with the reads. However, he offered me to send it for repair if the problem persists. So I will try to play normally with it to see if it really impacts my dayly playing and if so I will send it to repair. The guy told me that it was normal that not all the reads react the same way to the ammount of air pression. I will update you if something new happens with this melodica. Once again, Alan, thank you for your help.