I've the same problem.
It's so weird, because I didd all in the right way.
But, using firebug, in the code inside the class of the div I get:
class="itemContainer traduzione per l'editoria isotope-item"
without "width4"
as you can see here:
www.expotraduzioni.it/

where is the problem?
thanks to all

Hi, everything seems ok on your site, what´s the problem?

I fixed the problem by my self. It missed the "checked" in "show extra field" into the k2 item settings. According to me, the manual you should be more clear.
thanks
bye