How to have vertical and horizontal axis work at the same time

Ok,What i'm trying to do is to rotate an object in Roll(left/right) and Pitch(forward/back) axises like this:

function Update(){
    var horizontalRotation = Input.GetAxis("Horizontal") * rotationSpeed;
    var verticalRotation= Input.GetAxis("Vertical") * rotationSpeed;
    var rotationVector = Vector3(verticalRotation,0,-horizontalRotation);
    transform.Rotate(rotationVector * 0.1);
}

what happens is that, when for example the up arrow is pressed first, the left or right arrow does not make any change to the rotation anymore. Basically it only gets the first pressed button and ignores whatever comes after until the first button is released. how to fix this? thanks,

Works fine for me.

Is it possible you have a pretty old or dirt cheap keyboard? What could be happening is that your keyboard may be overloading the signals for the two keys, which was a common practice with older keyboards. Try pressing W and the left cursor at the same time for example instead of up and left, see if it rotates in both directions. If it does, it could well be your keyboard