For your draft invoices to go through to Xero, you need to update your Accounting Package Settings.
This can be done in the Web Portal by going the Settings Cog > Accounts > Invoices > scroll down to Accounting Package Settings (Please note: this might be called Xero Settings, depending on your accounting settings).
Ensure that the field 'Send Draft Invoices' is selected to say 'yes'. This will send draft invoices to your accounting package > Save at the bottom of the page.