Forms authentication keeps logging out

Hi
My forms auth is logging me out to login form after 20mins, even if timeout prop is set 360mins ??
web.config : 
    <authentication mode="Forms"  >
      <forms name=".TESTCK" timeout="360" protection="All" path="/" slidingExpiration="true" loginUrl="/CMS/Default.aspx" defaultUrl="~/Cms/Management/News/News.aspx"></forms>
    </authentication>
 
auth code (taken from msdn) :
...

 If VerifyUser(UserName.Text, Password.Text) > 0 Then

            Dim tkt As FormsAuthenticationTicket
            Dim cookiestr As String
            Dim ck As HttpCookie

&n ...