Displaying 'select' elements in a form table

Question

I have a table inside a form. I'm trying to display a 'select' form element in one of the table columns, but it doesn't display; it keeps getting rendered outside of the table. However, if I change the form element to 'checkboxes' or 'radios', it displays without problem.

I'm rendering like this: $output .= theme('table', $header, $rows);

where $header is an array for the table headings and $rows is an array of array (the table rows).

The rows array is built like this:

$row[] = drupal_render($form['fname'][$key]);
$row[] = drupal_render($form['lname'][$key]);
$row[] = drupal_render($form['days'][$key]);
$rows[] = $row;

What I want is for the final row ('days') to be a select box instead of a checkbox, but when I change it to select, its being displayed outside the table.

Any ideas? Thank you.

This is my theme function:

function theme_client_admin_nodes($form) {
  $has_posts = isset($form['fname']) && is_array($form['fname']);
  $header = array(t('First Name'), t('Last Name'), t('Packages'), t('Day'));

  $output = '';

  if ($has_posts) {
    foreach (element_children($form['fname']) as $key) {
      $row = array();
      $row[] = drupal_render($form['fname'][$key]);
      $row[] = drupal_render($form['lname'][$key]);
      $row[] = drupal_render($form['packages'][$key]);
      $row[] = drupal_render($form['days'][$key]);
      // $row[] = drupal_render($form['nodes'][$key]);
      $rows[] = $row;
    }
  }
  else {
    $rows[] = array(array('data' => t('No posts available...'), 'colspan' => '4'));
  }

  $output .= theme('table', $header, $rows);
  if ($form['pager']['#value']) {
    $output .= drupal_render($form['pager']);
  }

  $output .= drupal_render($form);

  return $output;
}