What is happening is that people are coming into your shop wanting to buy a certain product, and you do not have it?
Here would be a typical sale of products in a shop in order of sale quantity.
The stuff in blue everyone has that is the 70% of sales, the stuff in grey marked with a green arrow which makes up about 30% of sales, but it has more items.
Generally, it's because it is not worth stocking that you do not have it and these products are part of what we call the long tail.
Yet its not as simple as that, the interesting point is if you are trying to get new customers is that the people who are looking for items in grey do most of the searching, in fact, about 80% of the searches according to research by google. The people in the grey zone are looking.
So in modern retail it is considered that range is everything here as if people want an ABC voucher, and you do not have it, then they likely will go elsewhere to get it.