In case anyone else is looking for the answer, here's how I solved it:
Users were not being alerted when tagged in a message. This was due to how User Group Permissions were set when imported from vBulletin.
To fix this, go to User Group Permissions, and look under General Permissions. There is a setting for "Maximum mention alerts per message." On my forum, this was set at "0" for each user group. When I changed this to a number higher than zero, users would receive the alerts when tagged.