Drupal 8 Configuration API - cant access variables

I have made a simple example to practice using the configuration api in drupal 8. I created the following twitter_pull.credentials.yml file in the config/install folder:

oauth_access_token: "12345"
oauth_access_token_secret: "67890"
consumer_key: "54321"
consumer_secret: "09876"

and flushed my cache. In a custom block, I pulled in the configuration

$config = \Drupal::config('twitter_pull.credentials');

and when I did a dpm

dpm($config);

I did not get a null result. However, when I tried accessing a property:

drupal_set_message($config->get('consumer_key'));

now the result was null. Even when I tried using a configuration object which already exists

 $enabled = \Drupal::config('system.maintenance')->get('enabled');
dpm($enabled);

it still returned null. What am I missing here?

EDIT: If it helps anything here is the full code:

namespace Drupal\twitter_pull\Plugin\Block;
use Drupal\block\BlockBase;

/**
 * Provides a block for executing PHP code.
 *
 * @Block(
 *   id = "twitter_pull_tweets_block",
 *   admin_label = @Translation("Twitter Tweets")
 * )
 */
class TweetsBlock extends BlockBase {

  /**
   * Builds and returns the renderable array for this block plugin.
   *
   * @return array
   *   A renderable array representing the content of the block.
   *
   * @see \Drupal\block\BlockViewBuilder
   */
  public function build() {
    $config = \Drupal::config('twitter_pull.credentials');
    $enabled = \Drupal::config('system.maintenance')->get('enabled');
    dpm($enabled);
    dpm($config);
    drupal_set_message($enabled);
    return 'this is a block: ' . $config->get('consumer_key');
  }
}