form_alter ()

hello all,
i want to write a module who can interact with an another module (simplenews ).i have added a function in my module to alter the form of the old's form module. the problem is that i have to add two field in the new form .my function is:

function mynews_form_alter($form_id,&$form){
 
 if($form_id == 'edit-simplenews-block-form'){
 
 $form['com_connu']=array(
 '#title'=>t('comment avez vous connu '),
 '#type' =>t('textfield'),
 '#description'=>t('connaitre '),
 );
 
 $form['quel_boutik']=array(
 '#title'=>t('quel boutique aimeriez vous'),
 '#type'=>t('textfield'),
 '#description'=>t('quel boutik'),
 );
 }

 }

but when i see the block in my website the old form don't change i don't know why.
so i need some help.
thanks in advance.