Why isn't DefWindowProc() passing anything on?

I have subclassed a window using SetWindowSubclass(), and my message handler is successfully called, but when I call DefWindowProc() to pass the messages on for their original processing, no messages ever get through!

We are writing an extension to Marmalade (a kit for cross-platform mobile development). It allows for native extensions for specific platforms, and we're writing one for the Windows desktop build. We don't have direct access to the message loop, but can subclass to handle messages ourselves, however we don't seem to be able to pass the messages back to Marmalade for normal processing.

For example, even doing nothing in the callback but calling DefWindowProc() still doesn't work:

// Initialization:
const UINT_PTR gSubClassId = NULL;
DWORD_PTR subClassCBData = NULL;
SetWindowSubclass(gMainWindow, CadUtil_WindowCB, gSubClassId, subClassCBData);
...
// Message processing callback.
static LRESULT CALLBACK CadUtil_WindowCB(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam, UINT_PTR uIdSubclass, DWORD_PTR dwRefData)
{
    // Messages are correctly diverted here, but DefWindowProc() below isn't passing them on.
    return DefWindowProc(hWnd, message, wParam, lParam);
}

Any idea how this can happen?

Thanks,
Rob.

I still don't know why subclassing didn't work, but I was able to work around it by using hooks instead. So instead of using SetWindowSubclass() to capture messages and DefWindowProc() to pass them through, I now use SetWindowsHookEx() with WH_CALLWNDPROC and WH_GETMESSAGE to capture messages, and call CallNextHookEx() in the hook to pass messages through. This works where subclassing wouldn't.