Use the Manage Orders feature in Seller Central to search for the missing order. If the order appears in search results, make note of the purchase date and review your orders reports from that period. If the order is indeed missing from your orders reports or does not show up in search results, contact us and reference the missing order number.
As long as you have email order notifications enabled in your Account Settings, Amazon will send you a notice every time a new order is placed. If you are no longer receiving these emails, you should make sure that:
You can manually generate an order report in Seller Central at any time for the past 1, 2, 7, 15 or 30 days. For more information, refer to the Order Reports Help topic.
Seller Central is unable to generate orders reports beyond 30 days. To retrieve past data, you should archive your daily orders reports. To investigate specific orders that are older than 30 days, you can use the Search Orders feature in Seller Central. For more information on using this feature, refer to the Search Orders Help topic.
To quickly see a list of your orders, use the tabs on the Manage Orders page in Seller Central to see a list of all Undelivered, Pending, Shipped and Cancelled orders.
The best source for order information is the Orders Report, which you can download from Seller Central. An orders report is posted every 24 hours. You can also manually generate an orders report for the past 1, 2, 7, 15, 30, 60 or 90 days.
If email order notifications are enabled in your account settings, you will also be sent an email notice for each order when that order clears the Amazon Payments system.
See the linked help page for more information on this question.