Quickbooks PHP API: Refresh OAuth 2 Token Failed

I'm trying to access Quickbooks API using the PHP SDK but getting the following error:

Refresh OAuth 2 Access token with Refresh Token failed. Body: [{"error":"invalid_grant"}].

My Tokens seem to work for 24 hours but after that I receive the error above. Each time I call the API, I am saving my updated tokens to my database:

//Client ID & Secret
$qbClientId = $this->scopeConfig->getValue('quickbooks/api/qb_client_id', $storeScope);
$qbClientSecret = $this->scopeConfig->getValue('quickbooks/api/qb_client_secret', $storeScope);

//Retrieve currently saved Refresh_Token from DB
$qbRefreshToken = $this->scopeConfig->getValue('quickbooks/api/qb_refresh_token', $storeScope);

$OAuth2LoginHelper = new OAuth2LoginHelper($qbClientId, $qbClientSecret);
$accessTokenObj = $OAuth2LoginHelper->refreshAccessTokenWithRefreshToken($qbRefreshToken);

$error = $OAuth2LoginHelper->getLastError();

if($error) {
  throw new \Exception($error);
} else {
  // The refresh token and access token expiration
  $refreshTokenValue = $accessTokenObj->getRefreshToken();
  $refreshTokenExpiry = $accessTokenObj->getRefreshTokenExpiresAt();

  // Save new Refresh Token & Expiry to DB
  $this->configInterface->saveConfig('quickbooks/api/qb_refresh_token', $this->encryptor->encrypt($refreshTokenValue), 'default', 0);
  $this->configInterface->saveConfig('quickbooks/api/qb_refresh_token_expiry', $refreshTokenExpiry, 'default', 0);

  // The access token and access token expiration
  $accessTokenValue = $accessTokenObj->getAccessToken();
  $accessTokenExpiry = $accessTokenObj->getAccessTokenExpiresAt();

  // Save new Access Token & Expiry to DB
  $this->configInterface->saveConfig('quickbooks/api/qb_access_token', $this->encryptor->encrypt($accessTokenValue), 'default', 0);
  $this->configInterface->saveConfig('quickbooks/api/qb_access_token_expiry', $accessTokenExpiry, 'default', 0);

  return DataService::Configure(array(
    'auth_mode' => 'oauth2',
    'ClientID' => $qbClientId,
    'ClientSecret' => $qbClientSecret,
    'accessTokenKey' => $accessTokenValue,
    'refreshTokenKey' => $refreshTokenValue,
    'QBORealmID' => 'MyRealmID',
    'baseUrl' => 'Development'
  ));
}

So as you can see, on each API call, I'm using the refreshAccessTokenWithRefreshToken($qbRefreshToken) method to get new Refresh and Access Tokens and saving those to my DB for next use, however I still receive invalid_grant errors after 24hours.

Any ideas?

from Quickbooks PHP API: Refresh OAuth 2 Token Failed