"To" names don't show up in Outlook 2000

When I'm composing an e-mail, I click on the "To" button to bring up my list of contacts. Double-clicking on the name inserts the contact into the "To" box, but many times the name isn't displayed! There will just be a comma or semi-colon to indicate that something is there--but not the actual name. It doesn't happen for all contacts, but I haven't been able to figure out why it happens with some and not others. The "show as" detail box for the contact always has the person's name and e-mail address in it, so they should be displayed. The person gets the e-mail, but it sure makes it difficult for me to figure out who I've sent it to later! Any ideas? Thanks!