the students cannot enter the group

Invite students to register in your groups In order to invite your students to register in your groups, follow these steps: 1. Click on your username to open the user menu, and click "my students". 2. Click on "add group", enter your group name and click "save". 3. Click on "Invite students", next to the group name. 4. Copy the group code. 5. Pass the group code to your students, and tell them to click "students' access" and then "register as a student". They have to enter the group code and then their name, username and password. 6. You have to accept the students. For this, enter the main page of the site with the username and password. You'll see the students pending approval, and you can approve them. Remember that, after registering your students, you have to make workbooks with your favourite worksheets and assign them to the students.

He creado un grupo de clase y a un alumno en concreto no le deja entrar en él, la aplicación le dice que el código no es correcto pero está comprobado que es el que corresponde. ¿Cómo podemos solventar el problema? Gracias de antemano