I dont seem to get it

Here is what i did.
-attached UIForwardScroll to drag panel where slider is
-made target of UIForwardScroll to be a slider
-added this code to slider:
void OnScroll(float delta)
{
Debug.Log(delta.ToString());
}
when i use mouse scroll over slider nothing happens.
(for some reason only i can only make slider move by pressing on the thumb and dragging, other parts of the slider does not pick up presses although colliders are set properly)
what i am doing wrong, what is the right procedure to get this to work?
thank you very much!