Empty view cache file sometimes

I use file caching to cache the page output. Sometimes all the cache files are ok. Sometimes a few cache files are zero byte. These empty cache files are random (not consistently the same files).

I am using PHP 5.5.30 and Phalcon 2.0.9. The program is in development environment and I am the only user accessing the pages.

I cannot find similar problem in this forum or the internet so I think maybe I code my program wrongly. Thanks in advance for any suggestion.

Below are codes that I used to set up $di and the controller. The generated cache ID is a camel-cased file name without any folder name.

$di->set('viewCache', function() use ($config){
        //Cache data for one day by default
        $frontCache = new Phalcon\Cache\Frontend\Output(array(
            "lifetime" => 2592000
        ));
        //File backend settings
        $cache = new Phalcon\Cache\Backend\File($frontCache, array(
            "cacheDir" => $config->application->cacheDir,
            //"prefix" => "php"
        ));
        return $cache;
    });

/**
 * Setting up the view component
 */
$di->setShared('view', function () use ($config) {

    $view = new View();

    $view->setViewsDir($config->application->viewsDir);

    $view->registerEngines(array(
        '.volt' => function ($view, $di) use ($config) {

            $folder = $config->application->cacheDir.'compile/';
            if ( ! @file_exists($folder))
            {
                @mkdir($folder, 0777, TRUE);
            }

            $volt = new VoltEngine($view, $di);

            $volt->setOptions(array(
                //'compiledPath' => $config->application->cacheDir,
                'compiledPath' => $folder,
                'compiledSeparator' => '_'
            ));

            return $volt;
        },
        '.phtml' => 'Phalcon\Mvc\View\Engine\Php'
    ));

    return $view;
});

use Phalcon\Mvc\Controller;
use Phalcon\Mvc\View;

class FunController extends Controller
{
  public function indexAction()
  {
      $pageCacheID = $this->generatePageCacheID();

      if(!($this->view->getCache()->exists($pageCacheID))) {
          //generate content
      }

      //Save page to cache
      $this->view->cache(array('key' => $pageCacheID, 'lifetime' => 86400));
  }
}