Cognito with Lambda doesn't work at the first attempt, requires more

i have a function, which read a json with some users informations
i need to create a cognito user for everyone in the json:

const AWS = require("aws-sdk");
const cognitoidentityserviceprovider = new AWS.CognitoIdentityServiceProvider();
AWS.config.update({
  region: "eu-west-2"
});
const usersJSON = require("./users.json");
let saved = {
  Users: []
};

for (let o in usersJSON.Users) {
  let { name, surname, userClass } = usersJSON.Users[o];
  let emailToSave = `${nome}${cognome}${classe}@spalla.it`;
  let email = emailToSave.toLowerCase();
  let password = `Pwd${Math.floor(10000 + Math.random() * 90000)}`;

  let user = {
    nome: name,
    cognome: surname,
    classe: userClass,
    email: email,
    password: password,
    time: Date()
  };

  saved.Users.push(user);

  let params = {
    UserPoolId: cognito.userPoolId,
    Username: email,
    DesiredDeliveryMediums: [
      "EMAIL",
    ],
    MessageAction: "SUPPRESS",
    TemporaryPassword: password,
    UserAttributes: [{
        Name: 'nickname',
        Value: name
      },
      {
        Name: 'email',
        Value: email
      },
      {
        Name: 'custom:authCode',
        Value: "000000"
      },
      {
        Name: 'email_verified',
        Value: "True"
      },
      {
        Name: 'custom:userClass',
        Value: userClass
      },
      {
        Name: 'custom:role',
        Value: "user"
      }
    ]
  };

  cognitoidentityserviceprovider.adminCreateUser(params, function(err, data) {
    console.log("user created");
  });
}

the first time i run this function it does literally nothing, then the second time i run it it creates the two users successfully. ( note: this is only the specific part of the entire code, it was working perfectly some weeks ago.. but now it doesn't. it also includes at the end a function to send an email (nodemailer) with the users created and, as the creation, it works only at the third - fourth time. )

i'm using aws-serverless-express
what could be the problem? thanks in advance!