We are using a WMS software to manage all Shopify orders via API. When the orders are completed in the warehouse, the software will send a request to make as fulfilled. The question is some of orders was marked as fulfilled successfully, but some of them are failed. Sometimes those failed orders will be marked as fulfilled successfully after a few days with many tries. What is going on? Thanks.
The API request will return error messages if it fails. I would contact the software developers of the app you're using, they can debug and figure out why the fulfillment is failing.
That being said, sometimes random fulfillment failures happen with no good message, but setting up an auto retry after a few seconds usually solves it (in the cases when the error message is empty or a random 500 error).