If newsletter queue is not going to sent by default magento cron, try to send newsletter manually as following way. In some magento version there is problem of newsletter queue. It is not going to sent by default magento cron to all your subscriber. So your newsletter will not sent to all of your customer which are subscribe to your site.

But don't worry if this is happens with you can send your newsletter to all your customer manually as described below. 

i have found the page of newsletter queue which is

'app\design\adminhtml\default\default\template\newsletter\queue\list.phtml'
in this page getChildHtml('grid') ?> this function call the list body

which come from

'app\code\core\Mage\Adminhtml\Block\Newsletter\Queue\Grid\Renderer\Action.php'

add following code in that page but don't make any change in core file, override that file
in your local directory as

'app\code\local\Mage\Adminhtml\Block\Newsletter\Queue\Grid\Renderer \Action.php'

just copy and paste that file in this directory.

Now add the following code in line number 77 before this code $this->getColumn()->setActions($actions);

$actions[] = array(
'url'=> $this->getUrl('*/newsletter_queue/sending'),
'caption'=>Mage::helper('newsletter')->__('Send'),
'popup' =>true
);

Update: By default magento send only 20 recipients at time. if you want to change this number and want to send more recipients at a time change in following file.

app/code/core/Mage/Adminhtml/controllers/Newsletter/QueueController.php in this file change following line

$countOfSubscritions = 20;

to

$countOfSubscritions = 100; //change 100 with what ever number you want to change.

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