Mainly when synchronizing via HTTP, the data must not be from cache. Is there a way, like query parameter in URL to bypass cache of any kind in Drupal.
Something like request-url?cache=0
Check out "Cache exclude" module:
https://www.drupal.org/project/cacheexclude
Or if you want to do it from code you can add following line at top of specific pages:
$GLOBALS['conf']['cache'] = FALSE;