That means something in your CSS has a higher specificity on that element. Try adding the ID of an ancestor element in front of your selector. So for example if ul.nav is contained inside a div with the id of #left-toolbar or something like that, you would change:
ul li:hover .long
to:
#left-toolbar ul li:hover .long
Adding the ID will give this a higher specificity level than it has now. Whether or not that is enough to give it more specificity than whatever is overpowering it now or not, I don't know, cause I can't see your code. But it's something to try.
If you aren't using firefox with the firebug plugin, you should. It will show you what is the stronger CSS declaration for a particular element.