fail to draw bitmap correctly with Bitblt()

Question

Part of callback function of a child window:

case WM_MOVE:
	hdc = GetDC(hChild);
	hdcMem = CreateCompatibleDC(hdc);
	SelectObject(hdcMem, hBitmap2);
	BitBlt(hdc, 0, 0, width / 4, height / 4, hdcMem, or_x, or_y, SRCCOPY);
	UpdateWindow(hChild);
	ReleaseDC(hChild, hdc);
	break;

It only paints when it has been initialized but not been redrawn when I move the child window. or_x and or_y is two varibles changing when callback function is called. (And it is tested to be changing indeed)