Depending on the software you used to create your invoices, the 'Sent' status of your invoices might not be included in Billit. That means your invoices will be shown as not sent although you did send them in the original software.
To change the Sent status of your invoices go to the 'Invoices' menu and select the invoices you want to change. Then click 'Change sent status'. The invoices will now get the 'Sent' status.
If you need to change a lot of invoices, you can first filter them by the 'To send' status.