H.264 Encoder ProcessOut problem

I read data from IMFMediaSource,Then use H.264 Encoder to encode IMFSample,But some problem happened:

    hr=m_pVideoEncoder->ProcessInput(pSample);
    BREAK_ON_FAIL(hr);
    CComPtr<IMFSample> pSampleOut=NULL;
    hr = m_pVideoEncoder->ProcessOutput(pSampleOut);
    BREAK_ON_FAIL(hr);
    if(pSampleOut){
     LONGLONG duration=0;
     CComPtr<IMFMediaBuffer> pTestBuffer = NULL;
     DWORD wdBufferCount=0;
     hr=pSampleOut->GetSampleDuration(&duration);
     hr=pSampleOut->GetBufferCount(&wdBufferCount);
     hr=pSampleOut->GetBufferByIndex(0,&pTestBuffer);

     BYTE *pbBuffer=NULL;
     DWORD dwMaxLength=0;
     DWORD dwCurrentLength=0;

     hr=pTestBuffer->Lock(&pbBuffer,&dwMaxLength,&dwCurrentLength);  //pbBuffer's content is '\0',but dwMaxLength is 48000,and dwCurrentLength is 16000
     if(pbBuffer){
       hr=pTestBuffer->Unlock();
    }

I cannot solve this problem,  some one can help me,thanks!