The most common reason is due marketplace sku that is not mapped to a product id. In this state, Finale is hold in the order in a pending state, and all items in the order are not decremented until the mapping is made.
The quickest way to identify this issue is by reviewing the order details and noticing if there is a row with an item qty populated but the product id and description field are empty.
In the example provided below, the third item in the order has a product mapping error.
Please review this video at the 1:30 mark to learn how quickly address it.