keep in mind that php is a case sensitive language.
add the following code to the page to output the session variable value:
<?php
if(!session_id()) Session_start();
echo("Session txtgroup is equal to: |".$_SESSION['txtgroup']."|");
?>
I put the pipe character around the output to check for trailing or leading spaces. What does this print to the page? is it Admin with capitol A or lower case a? make sure to get the case correct in your rule.