VRWorks 360 Video nvstitchStartStitcher callback function not work

I want to check the progress of the stitching using callback function.
But it did not enter the callback function.

void cb_packidx(uint32_t packet_index,
                const nvstitchPayload_t* out_payload,
                const nvstitchAudioPayload_t* out_audio_payload,
                void* app_data)
{
    std::cout << packet_index << std::endl;
}


nvstitchResult
app::run(appParams* params)
{
......
    // Start stitcher
    RETURN_NVSTITCH_ERROR(nvstitchStartStitcher(stitcher, (nvstitchStitcherOnOutput)cb_packidx, nullptr));

......
}

I hava the same problem. did you solved it yet?