Order not showing in dashboard and using old order number

Hi

We had a couple of customers chase their orders today. They both ordered and paid via Sagepay and received an order confirmation for their products. However, 3 things:

1. The order did not show up in woocommerce dashboard
2. The order confirmation has allocated an old, already used order number which is completely unrelated, though the order details (product and quantity) are correct
3. No VAT was charged on their transaction.

We get about 25 orders a day so it is strange that something odd has happened to these two orders. We have had this before but put it down to a one of but clearly it is happening again.

Could anyone help please?

Thanks

David Gunning