[RESOLVED] DirectShow Video Capture Problem

I am new to DirectShow and I am developing a program which capture images from the usb pc camera.
As the Document of DirectShow did, I create my application and call initCaptureGraphBuilder(&pGraph,&pBuild) to initialize the point of IGraphBuilder and ICaptureGraphBuilder2
and then I call pGraph->AddFilter() to add the filter to the filter graph.
Up to now everything is ok and then I call pBuild->RenderStream() method to preview the video from my usb camera.
but the method returned a E_FAIL, to my suprise.
I don't know what's wrong with my program and
initCaptureGraphBuilder() and AddFilter() method all return S_OKs except the RenderStream() method.