If you see "Missing from GA" as a channel, there are a few scenarios that could be causing this classification.
Missing from GA is a literal term meaning that the order in the system does not have an associated session from Google Analytics associated with it. A small amount of orders are common to have Missing from GA as a classification. If you have a subscription service, you will see a large amount of orders with this Channel.
Causes of "Missing from GA"
1. Cookie blockers are enabled on the customer's browser. This does not allow GA to capture information about the session.
2. Orders are placed from the admin system by customer service, so there is no traffic associated with the order.
3. Subscription auto-reorders are automatically generated on a schedule, and therefore do not have a session associated with them
4. Retail POS orders would not have traffic
5. Amazon Orders