Hi @Todd Orme
Welcome to Atlassian Community!
Are you using a team-managed project or company-managed project? If you go to View all projects it will show you what type it is.
In company-managed projects you also have a permission scheme that controls what users have access to in the project, one of them being who can be assigned a work item. It could be that the user you are trying to assign the work item to is not in the right project role and is therefor not showing up in the list of assignable users.
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