If invoices are appearing on the Invoice Rejection Audit report, there is a detail line that describes the reason for rejection.
One reason that commonly occurs in relation to Sales Order is that the price is defined as T.B.A, of course, you cannot invoice an order if the price is not defined, the system does not assume the item to be F.O.C.
To remove this from the Invoice Rejection Audit report you will need to define a price for the items on the Order, it will then be possible to invoice.