Profile.IsAnonymous doesn't work

Hi folks

I have a big problem
There are some pages I want the users who are in my db see them.

I try to do an sign in page and it word fine

But when I wrote this line in the page load event It doesn't work at all
either when the user sign in

If Profile.IsAnonymous = True Then
    Response.Redirect ("Start.aspx")

It's allways true ??

Why ??

Thanks

Bye
...