This is most probably due to a couple of reasons....
1) Your budget limit has been reached. If you would like to get more vivos in your budget please ask your vivo admin at your school.
2) The student no longer exists in the system. This could be because the student has been blacklisted, removed or not yet entered into the system.
3) It could be that there is a technical glitch and if this happens please contact firstname.lastname@example.org and we will get it fixed immediately.