the value of the checkbox will only be sent if the checbox is checked, this is the way a checkbox works, if it is not checked, no value is sent.
the checkbox is also used by the insert record behavior as a trigger for inserting that record, if not checked, no record for that column is created. I would recommend using a radio group instead of a checkbox.
the other issue is because you are using recordset data in the insert behavior, the recordsets are configured to filter using query string variables. When the page first loads, the querystring variables are passed in, but the forms action does not pass them when it sends the form data, so the recordsets are not filtering correctly on post.
change the form tag:
<form action="" method="POST" name="attend_date">
<form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?><?php echo (isset($_SERVER["QUERY_STRING"]) && $_SERVER["QUERY_STRING"] != "")?"?".htmlspecialchars($_SERVER["QUERY_STRING"]):""; ?>" method="POST" name="attend_date">
this will include any existing querystring variables when the form is posted.