Data is missing from Query - mysql

Question

I have this query to combine dates in a date field of sales.
Once the person has more then 3 purchase dates and we run the query, they do not show up at all. If they have two or three it's not a problem, as soon as the have four dates to combine it just skips the line all together.
<?php
set_time_limit(0);
$server = 'localhost';
$login = 'xxxx';
$password = 'xxxx';
$db = 'xxx';
$table = 'xxx';
$filename = '/fakepath/file.csv';
mysql_connect($server, $login, $password);
mysql_select_db($db);
$fp = fopen($filename, "w");
$query = 'SET GLOBAL group_concat_max_len=15000';
mysql_query($query);
$res = mysql_query("SELECT t1.employee_number, t1.first_name, t1.last_name
, GROUP_CONCAT( DATE_FORMAT( t1.dates_of_purchase, '%m-%d-%y' ) SEPARATOR ' & ' ) AS Purchase_Date
, t1.Dept, t1.Location
FROM XXX AS t1
LEFT JOIN XXX AS t2 ON ( t1.employee_number = t2.employee_number
AND t1.last_name = t2.last_name
AND t1.QTY != t2.QTY )
WHERE t1.dates_of_purchase >= '2011-01-01'
AND t1.QTY >=1
AND t2.employee_number IS NULL
GROUP BY t1.employee_number, t1.first_name, t1.last_name
ORDER BY t1.employee_number
");
mysql_data_seek($res, 0);
while($row = mysql_fetch_assoc($res)) {
$line = "";
$comma = "xxx,";
foreach($row as $value) {
$line .= $comma . '' . str_replace('', '', $value) . '';
$comma = ",";
}
$line .= "\n";
fputs($fp, $line);
}
fclose($fp);`
This is the code and it works for the most part.
Why does it not return any rows for more then 3 purchase dates?
Here is the data:
Account employee_number first_name last_name dates_of_purchase Dept Location QTY Vendor id
ONTH30 652526 G HARRIPERSAD 2011-02-16 PWU NUCLEAR SUPPLY CHAIN 1 MSS 1836
ONTH30 652561 AJJ FELS 2011-09-15 PWU NOSS 1 MSS 4319
ONTH30 652561 AJJ FELS 2011-06-23 PWU NOSS 1 MSS 8898
ONTH30 652561 AJJ FELS 2011-08-18 PWU NOSS 1 MSS 8938
ONTH30 652526 G HARRIPERSAD 2011-11-16 PWU PICKERING 1 MSS 7231
ONTH30 652561 AJJ FELS 2011-12-08 PWU NOSS -1 MSS 8141
ONTH30 652561 AJJ FELS 2011-10-13 PWU NOSS 1 MSS 9055
The data missing from the output is employee number 652561.
It does not get output to the CSV file it's skips it all together for some reason.
The output should look like this:
opg, 652561, AJJ, FELS, 10-13-2011 & 12-08-2011 & 08-18-2011 & 06-23-2011 & 09-15-2011 , ESR, PICKERING A