Hi Heidi,
Users are not automatically subscribed to work objects in the system by design, this is so that users are not overwhelmed with notifications that they don't necessarily want to see. If Workfront subscribed the entire project team to all notifications on all tasks / issues / project updates, peoples inboxes could become flooded with notifications that aren't always relevant to them.
If a user has a particular interest in a work object in the system they have the ability to subscribe themselves, or if someone wants to ensure that another user gets a notification they should tag that user in to the update. This is the best practice in my experience.
One setting that you could enable (if you haven't already) is the 'Someone comments on one of my work items' notification setting on a users profile. Having this enabled will ensure that the user receives all updates on tasks / issues that they are assigned to.
Best Regards,
Rich.