JAVA robot模拟键盘失效

我用程序模拟锁屏无效啊WIN+L,但是WIN+R,,,,WIN+Q都能用,就是模拟WIN+L无效,求解决
Robot robot = new Robot();

        robot.keyPress(KeyEvent.VK_WINDOWS);
        robot.keyPress(KeyEvent.VK_L);


        robot.keyRelease(KeyEvent.VK_WINDOWS);
        robot.keyRelease(KeyEvent.VK_L);

用另一种方式实现的锁屏：
Runtime.getRuntime().exec("RunDll32.exe user32.dll,LockWorkStation");