the resortID querysting is present when the page loads, but it is not included as a part of the forms action, so when the form posts, it is no longer present
change:
<form action="resortunitamenities_Insert.php" method="post" name="WADAInsertForm" id="WADAInsertForm">
to:
<form action="resortunitamenities_Insert.php?resortID=<?php echo(isset($_GET['resortID'])?$_GET['resortID']:""); ?>" method="post" name="WADAInsertForm" id="WADAInsertForm">