PA Cycle Billing will not automatically bill the Purchase/Material cost categories

Hi Folks

My PA version is 12 build 1729.   I'm using Cycle billing and I'm creating a Billing document per project.   My selling price is zero for everything.   The system will bill all the cost categories in a project but will not automatically tick the lines if the cost category is a Purchase/Material type.

Would anyone know why this type of Cost Category will not automatically be ticked?

Thank you