Thank you so much for reaching out to us. I can certainly explain this.
ShippingEasy will only be able to sync unshipped orders from your Amazon account. As soon as you marked these as shipped, we would not longer have been able to import these orders.
If these are not Prime orders and you'd like to manually upload your orders via CSV, you can do so using these steps. Please keep in mind if you do this, we will not be able to send updates to Amazon regarding these orders.