Sent invoices not marked as sent

Hi,

I have a couple of invoices I sent last week. In the column these were marked as sent at the time. I have the CC'd copy of the sent invoice in my inbox.

Now the sent column is no longer ticked. As if they are unsent.

Which is not good as obviously I need to track this so I don't look like a complete idiot by resending an invoice.

Any got any ideas?

Thanks

Mark