That could be due to two things. 

  1. Please make sure that paypal IPN is set up correct as follows
  2. If that doesn't work, it could be due to the fact that your server cannot validate SSL certificate. Open the file includes/library/paypal.class.php and go to line 96 to find the code below and replace true by false

curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, true);