I agree with Smokin on placement of butts on racks and the location within the FEC 100. I tend to switch the top rack with the lower rack about halfway through the cook also, but that is just me.
One other thing, I find my FEC actually cooks better with a full load of meat than cooking less than that. Invariably, some of the butts finish faster with a full load in the smoker. So keep an eye on that.