Order not sync from OMS to WMS.
If the specific order is not appeared in WMS, user required to login to OMS, go-to order module, under "New" status, click the drop-down panel to check if the order are located in any of the "Not ready to fulfill" status. The issues required to be resolve and it will move to "Ready to fulfil" automatically, and then will sync to WMS for process.
To process Not Ready To Fulfill orders - https://support.selluseller.com/support/solutions/articles/44001798663-how-to-solve-not-ready-to-fulfill-orders-problem-orders-
During the first time of Marketplace or Webstore connection with OMS, OMS will fetch 90-days of historical order into OMS order module for reference use. These order are probably processed completed and it will not be pushed to WMS to process.