A lot of forums go through slow times when they are first being started (and even larger forums, but it is less noticeable in the large ones), this is also the reason that they die out. When you hit a slow time in your forum life, DO NOT just let your forum die out, keep posting, even more, and advertising as much as possible. Is need be, start a contest or, as a last resort, start hinting you are looking for staff to get activity up. But never let your forum die out.