It is a normal situation, if you activated your subject today. Keep in mind students and teachers have instant access, even though you don’t see them in the Participants list.

Names and surnames of students and teachers will appear in the Participants list:

• if the same day, they access to the classroom, or,
• the next day the list will be complete, because a massive  synchronization is executed during the night.

It’s been over 24 hours and I only see a part of the students in the classroom

In this case it could be that Sigma has more than one teaching group enrolled in the subject and, for some reason, only one of them is registered in the classroom.

Review the information provided by Sigma and, if there are one or more unregistered groups in the classroom, assign the missing ones through Moodle Classroom Management (on the Virtual Campus Board). You have more information here.