Request User drop down is missing users

I am having an issue where the Request User drop down in a New Incident is missing several of our users that are in the End User list. I have not applied any filters or made any changes to the New Incident form so I am not sure what is causing this. Does anyone have anything that they think I should check? Steve P.S. - I just noticed that I put the wrong field name on this. I have corrected it.