You should add missing invoice only if there is a case where client has accounted, where as vendor has not shown in his outward return.
With respect to invoice shown by vendor, but we are not showing in our inward return the reason may be the following :
1) Booked in subsequent month - if that is the case, keep the status as "PENDING" for this month and in subsequent month you can accept
2) If you have missed to book the invoice, then you should pass entries in the books of accounts and credit shall be taken in the month in which entry is passed, and the status can be "ACCCEPTED"