Failed to parse UFF use UFF Parser

When I use the uff parser to parse uff file, I got failed, and my tensorRT version is TensorRT-5.1.2.2.
int maxBatchSize = 1;
auto parser = createUffParser();

/* Register tensorflow input */
parser->registerInput("input_1", Dims3(256, 256, 3), UffInputOrder::kNHWC);
std::cout << "解析 UFF 文件结束 " << std::endl;
parser->registerOutput("model_2/conv2d_10/Sigmoid");
parser->registerOutput("model_2/conv2d_15/Sigmoid");

ICudaEngine* engine = loadModelAndCreateEngine(fileName.c_str(), maxBatchSize, parser);

if (!engine)
    RETURN_AND_LOG(EXIT_FAILURE, ERROR, "Model load failed");

/* we need to keep the memory created by the parser */
parser->destroy();

execute(*engine);
engine->destroy();

I see that when parse uff file ,it needs to be UffInputOrder::kNCHW format. but When I only use one output node, it works well. 
Anyone can tell me why? Thanks