Create a shipment API call - no valid responses

I had asked this question 3 days ago and no one responded.

We have tried using the sandbox and spent 10-15 hours trying to get a valid response on the "create a shipment" api call. We have tried thousands of combinations of data and no matter what we do we never get anything other than a 202 response. The 202 response also doesn't show us why no couriers are providing rates.

Happy to provide our API code if there is anyone who can help us with this.