Mqtt topic subscribe issue


GSM devise gets connected to MQTT server & also accepts the request connection, but while subscribing to topic, i am getting error every time e.g. subscribe denied.
I am using standard mqtt libraries for mqtt protocol.

May be some subscribe topics are denied. Like # is not allowed on some brokers for security purposes.
If allowed then check if packet format is correct. May be byte formation is wrong or QOS values might not be supported