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It's due to the insufficient power supply. I used three batteries one for motors,one for bt module,one for Arduino mini pro. It worked for me but it becomes too heavy to lift up. My suggestion is to try using three batteries as I did. I hope it works if not there might be problem in Bluetooth module
If hc module works fine,then the problem is in dumping the program into Arduino.Make sure the program dumped successfully.As per my knowledge,when it starts receiving commands the motors consume more power which turns off Bluetooth module automatically.so I suggest u to use different power supplies for motors and Bluetooth module.