I have a problem with the following implementation of hook_cron in Drupal 6.1.3.

The script below runs exactly as expected: it sends a welcome letter to new members, and updates a hidden field in their profile to designate that the letter has been sent. There are no errors in the letter, all new members are accounted for, etc.

The problem is that the last line -- updating the profile -- doesn't seem to work when Drupal cron is invoked by the 'real' cron on the server.

When I run cron manually (such as via /admin/reports/status/run-cron) the profile fields get updated as expected.

Any suggestions as to what might be causing this?

(Note, since someone will suggest it: members join by means outside of Drupal, and are uploaded to Drupal nightly, so Drupal's built-in welcome letters won't work (I think).)

<?php
function foo_cron() {
    // Find users who have not received the new member letter, 
    // and send them a welcome email

    // Get users who have not recd a message, as per the profile value setting
    $pending_count_sql = "SELECT COUNT(*) FROM {profile_values} v 
     WHERE (v.value = 0) AND (v.fid = 7)"; //fid 7 is the profile field for profile_intro_email_sent
    if (db_result(db_query($pending_count_sql))) { 
    	// Load the message template, since we 
    	// know we have users to feed into it.
    	$email_template_file	= 	"/home/foo/public_html/drupal/" . 
    								drupal_get_path('module', 'foo') . 
    								"/emails/foo-new-member-email-template.txt";
    	$email_template_data 	= file_get_contents($email_template_file);
    	fclose($email_template_fh);
    	//We'll just pull the uid, since we have to run user_load anyway
    	$query = "SELECT v.uid FROM {profile_values} v 
    	WHERE (v.value = 0) AND (v.fid = 7)";	 
    	$result = db_query(($query));
    	// Loop through the uids, loading profiles so as to access string replacement variables
    	while ($item = db_fetch_object($result)) {
    		$new_member = user_load($item->uid);
    		$translation_key = array(
    			// ... code that generates the string replacement array ...
    			);
    		// Compose the email component of the message, and send to user
    		$email_text = t($email_template_data, $translation_key);
    		$language = user_preferred_language($new_member);  // use member's language preference
    		$params['subject'] = 'New Member Benefits - Welcome to FOO!';
    		$params['content-type'] = 'text/plain; charset=UTF-8; format=flowed;';
    		$params['content'] = $email_text;
    		drupal_mail('foo', 'welcome_letter', $new_member->mail, $language, $params, '[email protected]');
    		// Mark the user's profile to indicate that the message was sent
    		$change = array(
    			// Rebuild all of the profile fields in this category, 
    			// since they'll be deleted otherwise
    			'profile_first_name' => $new_member->profile_first_name,
    			'profile_last_name' => $new_member->profile_last_name,
    			'profile_intro_email_sent' => 1);
    		profile_save_profile($change, $new_member, "Membership Data");
    	}
    }
}