MediaRecorder not recordng audio consistently

Question

I have tried to record audio from MIC using following code:

MediaRecorder recorder= new MediaRecorder();
recorder.reset();
recorder.setAudioSource(MediaRecorder.AudioSource.MIC);
recorder.setOutputFormat(MediaRecorder.OutputFormat.AMR);
recorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);
recorder.setOutputFile(path);

if(recorder!=null)
{
    recorder.prepare();
    recorder.start();
}

Some time this code records audio perfectly, but some times recorded file only has 6 bytes.

I don't understand why same code not works every time.

Please help me to solve this problem.