This can happen if a Nominal account is changed to a Bank account. Any transactions entered prior to the change will not be marked as Bank transactions and will not show in the Bank Reconciliation.
In order for them to show, there are two possibilities depending on whether there are a small or large number of transactions:
Small number of transactions: Reverse the transactions and re-input them.
Large number of transactions: An update can be applied to the database to reset the status of the transactions, so they then show up in the reconciliation. To get assistance with this, our support team will need to investigate, run scripts or liaise with the hosting or development teams. Any Issues with contacting us, please see the following Get support in Dimensions.