Whenever an admin tries to login on the admin login page they enter in their credentials, the page reloads and nothing happens. Why can't they login even though I'm sure their password and username are correct?
This generally means that the admin is in the API Administrators security group. This group is for "virtual" admins when API calls are made, so it does not allow access to the back-end of Luminate. Removing your admin from this group should fix the problem.