You know what sells in your shop.
However, do you know if you are selling all the items you could? Do you know whether other products would not sell better?
Your suppliers are a well of knowledge.
Ask them for a best seller list of their products and suggest they recoup the information with your orders from the past 12 months.
You will immediately identify which products you should be offering and which items are specific to your shop. You can then research the reasons why and target the customers with extra lines.