User can only change flag on certain Inbox items

[mgaffney]

I have a user who, for some odd reason, is only able to change the Flag
Status on select messages in her Inbox. She checks another person's Email on
their behalf, and she thought that maybe those messages were the ones that
couldn't be modified, but I wondered if there might be some other reason this
might be happening?

Thanks to anyone who can help me with this!