Affiliate sign up doesn't work

Hi Team

I see that we get visitors by our affiliates and I know they sign up, but I can't see it in the overview:
=> http://screencast.com/t/aMhTsUcpBVRw

(For translation: 'Aanmeldingen' = 'Sign ups')

Do you know why?

I turn on the supporter add-on.

Thank you in advance for your help!

Stijn