The Monthly Product Usage Report does not show assembly items and some flat rate tasks.
Cause of Issue:
The Monthly Product Usage Report looks at the InvenAct table to get its information. Assembly items are not true inventory items. They are merely a collection of stock and serialized parts, so when one is used on an invoice all the subassemblies will show in the InvenAct table but the assembly itself will not. Most flat rate systems import flat rate tasks as an assembly part, causing them not to appear on the report.
Use the Sales Item Summary to get this information instead. It can be found under the Sales menu in the Reports section.