ASP.NET - some times url authentication is not working..

hi..i have specified url authentication for the following pages ..but its not wording some time..but some time works..
what will be the problem...
<authentication mode="Forms">

    <forms loginUrl="index.aspx">
    </forms>

    </authentication>

 <location path="Registration.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "   />
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="PublicRegistration.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "   />
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="Public_view.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "   />
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="Public_listview.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "   />
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="Public_FullView.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "/>
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="Public_EditFullView.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "/>
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="mastersubcaste.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin"/>
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="mastermothertongue.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "/>
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="Mastercaste.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "/>
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="listview.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "/>
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>

  <location path="changepassword.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "/>
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="Admin_view.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "/>
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="Admin_FullView.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "   />
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <!--<location path="FullView.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "   />
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>-->
  <location path="website_FullView.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "   />
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="Website_changepassword.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "   />
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="Website_Editlistview.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "   />
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>
  <location path="Website_view.aspx">
    <system.web>
    <authorization>
      <allow roles=" admin "   />
      <deny users="?"/>
    </authorization>
    </system.web>
  </location>

May be you are not getting the right Userrole or Username in to your session or class objects and unble to verify the user or role against the config setting you mentioned.

Thanks

for all url which are specified here...

try this way if it give you result: