Hello there, TerryGH.

I've replicated your concern and saw some of the customers has zero amount under Cost of Goods Sold. Some of them have amounts. I've dug further and found out that if it's zero, it means they no longer have balances. You can click on the amount to open the Transaction report. 

You can refer to the screenshots below.

Let me know if you have additional questions by leaving a comment below. Have a great day!