Products in the Bestsellers block are shown according to sales statistics. The Bestsellers side box is displayed if there are orders in your store that have Completed or Processed statuses.
On the home page Merchium displays a list of all bestsellers. When a visitor enters a category the Bestsellers section shows best selling products according to sales statistics for  this particular category. So if there are no bought products in this  category the Bestsellers side box disappears.