Unless they have a sign up saying "all food guaranteed to always be available" or somesuch, any eating establishment has the right to decline to serve any item. Mayb the machine is broken, maybe it's being cleaned, maybe they don't have all the ingredients, maybe they're just understaffed and certain items take too long to make to adequately serve all the customers. It really doesn't matter, they have every right to do this... indeed, there might even be laws that state the machines must be cleaned every certain time period.

Bottom line: getting a shake when you want it is not a constitutional right. Make enough of a pest out of yourself and you'll probably find yourself banned from the whole restaurant for good.