If you have ran your vat return and clicked on Mark as reported but unticked the option to send to HMRC you can undo the mark as reported option, enter more invoices and re-run your return.
Once the Marked as reported has been selected all entries relating to that specific return are flagged as reconciled and will not be included in any other return.
You can undo this option by locating the return in Reports > VAT > Archive. Locate the return and click on the far right hand side icon Undo mark as reported this reverses the process and releases all transactions from the report and removes the reconciliation flag against the transactions. Your vat return can then be ran again for the same period and include any late entries and then reported to HMRC.