This was resolved with the help of Jason. Thanks again Jason!
Ray, the code should have worked as we had it but for some reason changing this:
$_POST['WADAfigure_lines_series.Series_Id'] = $row_WADAfigure_lines['series.Series_Id'];
$_POST['WADAfigure_lines_series.Series_Id'] = $row_WADAfigure_lines['Series_Id'];
did the trick even though the column is series.Series_Id in BOTH recordsets.
Anyway, thanks again for your help guys. Just one more issue from completing all the back end code and can try to make this mess look pretty.