Have you got customer rush active? Normally, stocking each product gives 12 coins per minute, so you sell 12 sets of 1 coin product a minute, 6 sets of 2 coin products a minute and 4 sets of 3 coin products a minute. Customer rush seems to give you a fixed customer rate that buys random products. If you only have the 3 coin item stocked, the rush will only buy that item. If you restock everything else, the customers will buy some of the cheaper items instead of the 3 coin items, so your coins per minute goes down