This is likely because the event is secure but no roles have been granted access.

1. Go to onMessage > Content Types > Events and select the gear to edit the event settings next to the event in question.
2. Under Access check to see if the event is marked a Public or Secure
3. If the event is public, it can be viewed by anyone. If the event is secure, only select roles will have access.

User-added image

4. If the event is secure, make sure that the correct roles are selected.

Note: If no roles have been selected, no users will be able to view the event.. This includes parents and school faculty/staff.