why cannot receive data in ProcessOutput

I have received data from IMFMediaSource in IMFTransform::ProcessInput, after I process the data then I should receive it in ProcessOutput.

But It seems not work. when I get the IMediaBuffer in ProcessOutput, the buffer size is zero.

HRESULT OggDecoder::ProcessInput(DWORD dwInputStreamID, IMFSample  *pSample, DWORD dwFlags)
{
    if (pSample == NULL)
    {
        return E_POINTER;
    }
    if (!IsValidInputStream(dwInputStreamID))
    {
        return MF_E_INVALIDSTREAMNUMBER;
    }
    if (dwFlags != 0)
    {
        return E_INVALIDARG; 
    }
    EnterCriticalSection(&m_critSec);

    HRESULT hr = S_OK;
    LONGLONG rtTimestamp = 0;

    if (!m_pInputType || !m_pOutputType)
    {
        hr = MF_E_NOTACCEPTING;   
    }
    else if (HasPendingOutput())
    {
        hr = MF_E_NOTACCEPTING;  
    }
    if (SUCCEEDED(hr))
    {   
       hr = pSample->ConvertToContiguousBuffer(&m_pBuffer);  // I can get the data buffer here!!!
    }

    if (SUCCEEDED(hr))
    {   
        hr = m_pBuffer->Lock(&m_pbData, NULL, &m_cbData);
    }

    if (SUCCEEDED(hr))
    {
       
        if (FAILED(pSample->GetSampleTime(&rtTimestamp)))
        {
            rtTimestamp = INVALID_TIME;
        }

        m_StreamState.TimeStamp(rtTimestamp);

        hr = Process();// do something
    }

    LeaveCriticalSection(&m_critSec);
    return hr;
}

HRESULT OggDecoder::ProcessOutput( DWORD dwFlags, DWORD cOutputBufferCount, MFT_OUTPUT_DATA_BUFFER  *pOutputSamples, DWORD  *pdwStatus  )
{
  if (dwFlags != 0)
    {
        return E_INVALIDARG;
    }
    if (pOutputSamples == NULL || pdwStatus == NULL)
    {
        return E_POINTER;
    }
    if (cOutputBufferCount != 1)
    {
        return E_INVALIDARG;
    }
    if (pOutputSamples[0].pSample == NULL)
    {
        return E_INVALIDARG;
    }
    EnterCriticalSection(&m_critSec);
    HRESULT hr = S_OK;
    DWORD cbData = 0;
    IMFMediaBuffer *pOutput = NULL;

    if (!HasPendingOutput())
    {
        hr = MF_E_TRANSFORM_NEED_MORE_INPUT;
    }

    // Get the output buffer.
    if (SUCCEEDED(hr))
    {
        hr = pOutputSamples[0].pSample->GetBufferByIndex(0, &pOutput);   // I cannot get the buffer here!!!!!!!
        hr = pOutput->GetMaxLength(&cbData);    // and cbData will be zero!!!!
    }
    if (SUCCEEDED(hr))
    {
        hr = InternalProcessOutput(pOutputSamples[0].pSample, pOutput);
    }

    if (SUCCEEDED(hr))
    {
       // do something
    }    

    SafeRelease(&pOutput);
    LeaveCriticalSection(&m_critSec);
    return hr;
}