Hi,
Is it happening for a Single Invoice or many?
Check the pricing date at sales order level whether it is within the validity and same effected in Invoice.
If it is a Order related Billing then Check the Copy control - VTFA for Pricing Type for the particular item category - should be "D" -Copy Pricing elements unchanged
If it is a Delivery related Billing then check the Copy control -VTFL for Pricing type for the particular item category and provide us the screen shot for it.
Regards,
SRK