Offtoa uses many of your stated assumptions to compute the COGS, so any of these could cause a row of your COGS on your income statement to be all zero:
Related Questions:
How does Offtoa calculate cost of goods sold?
Where do I enter/change raw materials?
How do I relate raw materials to products I sell?
Why do I have zero COGS for just one product?
How do I change the price of a raw material?