Why won't Safari ask me to autosave my passwords for some websites?

I have it checked in my preferences to autofill passwords and things like that. Some websites it will ask me Do you want to save your Username and password?(or however it words it) and I agree and it’s fine. However on some websites it will not prompt me to allow it to save the information. Fluther being one of them, yahoo mail and many others. Does anyone know why? I am using the latest version of Safari and Snow leopard if that matters at all.