Leonardo keyboard/mouse control not working

I recently got myself an arduino leonardo, which for the biggest part works without any problems. I did have to change a file in order to install the drivers (which is described here Leonardo drivers for W7 - Other Hardware Development - Arduino Forum).

But now I want to use it to control my mouse, but it doesn't seem to be able to control anything.

The code I last used was:

void setup() { 
  Keyboard.begin();
}

void loop() {
  Keyboard.write('u'); 
  delay(1000);
}

It doesn't work at all though, the mouse commands do not work either..

What could cause this problem?

Edit: The "TX" light does light up when it should be typing keys.

Well, if I just plug it into my laptop it works fine. So I guess it's a driver issue? I have never manualy installed the drivers there though..
Could anyone help me with this?

Although I wasn't using a USB HUB or something similar, changing the USB port seemed to do the trick.