A supplier's lead time changes, a new factory, a shipping delay, a different minimum order quantity negotiated last month, and Demand Planner keeps using whatever lead time it was last told, not what's actually true right now. A planned promotion or a known seasonal spike that hasn't happened yet in the sales history doesn't show up in a purely historical forecast either, which means the suggested reorder quantity can be right for last year and wrong for what's about to happen.
Multi-channel demand is another blind spot if some of your sales history sits outside Brightpearl, a marketplace or storefront that feeds in with a lag, or doesn't feed in at all. Demand Planner can only forecast off the sales it can actually see.