Realm Password: "Basic" Works, but not "Digest" or "Kerberos"

I am trying to make certain files password protexted on my web server. When I access the file via URL, and have the realm password set to "Basic", I can access them with no problems. However, if I try to increase the protection to "Digest" or "Kerberos", it doesn't work. I'll enter my username/pswd and submit, and the box pops up again and again and again. Any ideas about what is going on?

Thanks,

Ally