Business central - my setup is not posting to material cost (COGS) correctly

Hi we use Business central and our costing is FIFO.  we resell subscription based items to customers that gets billed annually and some are billed monthly.  We buy these items from vendor and we get billed monthly.  As I post the purchase invoices for items we purchase, I expect the cost of material GL account to reflect the cost of items, however, it doesn't seem.  I see the COGS account is specified for the gen. product posting group correctly along with purchase account.  what am I missing here?