XHR and php Progress bar, working on local, not on remote - php

Question

More explanation here :
Im importing products to my DB, and everytime a product is imported I display the percent calculated earlier:
//Progress bar
set_time_limit(0);
ob_implicit_flush(true);
ob_end_flush();
sleep(1);
$p = ($i/$count_product)*100; //Progress
$response = array( 'success' => true , 'message' => $p . '% ','progress' => $p);
$i++;
echo json_encode($response);
On my local everything is working fine :
Every time php echo json_encode() I get on XHR a new state 3, and display the response.
Code below:
var xhr = new XMLHttpRequest();
xhr.previous_text = '';
$param = 'id='+$(this).find('input').val();
xhr.onerror = function() {console.log(xhr.responseText) };
xhr.onreadystatechange = function() {
//try{
console.log(xhr.readyState);
console.log(xhr);
console.log(new_response);
if (xhr.readyState == 4){
var new_response = xhr.responseText.substring(xhr.previous_text.length);
var result = JSON.parse( new_response );
if (result['message'] == '100' ) {
$('.progress_bar').html('<div><?=$this->translate->_("Imported succefully")?></div>');
$.when($('.full_screen_layer').fadeOut(2000)).done(function() {
location.reload();
});
}else{
$('.progress_bar').html('<div style="color:red">'+result['message']+'</div>');
setTimeout(function(){
$.when($('.full_screen_layer').fadeOut(2000)).done(function() {
location.reload();
});
}, 2000);
// alert('<?=$this->translate->_("Please try again")?>');
}
}
else if (xhr.readyState > 2){
var new_response = xhr.responseText.substring(xhr.previous_text.length);
var result = JSON.parse( new_response );
if (result['success'] && result['end'] == undefined) {
$('.progress_bar').html('<div>'+result['message']+'</div>');
xhr.previous_text = xhr.responseText;
}else{
$('.progress_bar').html('<div style="color:red">'+result['message']+'</div>');
}
}
//}
// catch (e){
// alert("[XHR STATECHANGE] Exception: " + e);
// }
};
xhr.open("POST", action, true);
xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
xhr.send($param);
But I'm trying to understand why it's not working on the server ( XHR state 3 is fired only once, at the end of the request ( so the json is malformed )
Here are the headers:
This just break my mind, if someone have an idea, even a bad one xD
(Btw there is a proxy on the server : VIA:1.1 Alproxy ) it could stop the response untill it end and then send it alltogether ?