if your classes in app/library are namespaced, you should register them: https://docs.phalcon.io/en/latest/reference/loader.html#registering-namespaces
but maybe the loader just can't find the ap/library directory. Bellow a classic MVC app structure:
app/
config/
config.php
service.php
loader.php
controllers/
models/
.....
then:
// in config.php:
return new \Phalcon\Config(array(
....
    'application' => array(
        'controllersDir' => __DIR__ . '/../../app/controllers/',
        'modelsDir'      => __DIR__ . '/../../app/models/',
        'viewsDir'       => __DIR__ . '/../../app/views/',
        'pluginsDir'     => __DIR__ . '/../../app/plugins/',
        'libraryDir'     => __DIR__ . '/../../app/library/',
        'cacheDir'       => __DIR__ . '/../../app/cache/',
        'baseUri'        => '/',
    ),
.....
// in loader.php:
$loader = new \Phalcon\Loader();
/**
 * We're a registering a set of directories taken from the configuration file
 */
$loader->registerDirs(
    array(
        $config->application->controllersDir,
        $config->application->modelsDir,
         $config->application->pluginsDir,
          $config->application->libraryDir
    )
)->register();