I'd check to see if you have this report: 'Invoice Item Detail with Pricebook Information' - If you have the ability, this could also be created with a custom report using the Accounting Type > Invoice Items report template.
Once ran, you could drag the Item name or Item Code column into the purple box to group it. This would show you where an item was used, the invoice, etc. If Grouped it will show a sum at the bottom of the Item Quantity column.
If for some reason you don't see this report you may need to have an Admin check your reporting permissions.
