$this->input->post can't find my value from a form

[eluser]kris10[/eluser]
I have a form with a field user_name

when I click ok I want to put what I have on my field on a variable

$u = $this->input->get_post('user_name');
echo "user ".$u;

It works on local developpment but on my productiv server I have nothing on my variable.

I uploaded exactly the same code execpt config parameter that I changed accordingly ( mysql .. )

I am searching this for hours..; Just ideas ...