Have you tried forwarding the messages to oldProc(), instead of DefWindowProc(), in your WindowProc() ???
Probably GLUT has installed its own WNDPROC and you are kicking that one out of chain!
Also I think that obtaining the window handle with FindWindow() where only the title is specified, is not really reliable. Normally you should have the HWND of the window that you created yourself in a more direct way.
extern GLUTwindow *__glutCurrentWindow;
__glutCurrentWindow->win // <-- should be HWND on Win32
__glutCurrentWindow->renderWin // <-- should be HWND on Win32