Hi again James
Thanks for confirming that there are no specific firewall rules in Azure and the networking settings for the Form Recognizer service are configured to allow all networks, still, this might have happened due to various reasons like Api, Double-check that you're using the correct API key and endpoint url also Confirm that you are using the correct HTTP request method, these are a few things to consider, I know these are basic things still its good to double check.
If you're still unable to resolve the problem, I recommend contacting Azure Support they can provide expert assistance and help diagnose the issue more thoroughly.
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