Check the error passed in to the block, probably you are sending to many request and the rate limit is reached.
There is rate limit for reverseGeocodeLocation:completionHandler:
Geocoding requests are rate-limited for each app, so making too many requests in a short period of time may cause some of the requests to fail. When the maximum rate is exceeded, the geocoder passes an error object with the value kCLErrorNetwork to your completion handler.