HC-Sticky stops working after the page has scrolled so far

Bit of a newbie question here so please be gentle!

I have used HC-Sticky to create a sticky sidebar menu on my joomla webiste (http://www.youngmelksham.org.uk). An example can be seen on every page except the home page.

The menu stops 100px from the top as it is supposed to, but then after you have scrolled down the page some more the menu scrolls off the page.

It's like it works for about 150px then stops.

I use the following code to call the HC-Sticky

jQuery(document).ready(function($){
    $('#Mod{$module->id}').hcSticky({
        top: 100,
    });
});

Any advice would be really gratefully received.