@mx1 , echoing Giri that we’d definitely be able to help better with a code snippet.
I did have an experience before where webhooks specifically were not firing from a 3rd party, but the endpoint directly did work via Postman. We had come to the conclusion that it was possibly the IP addresses of the 3rd party servers being temporarily blacklisted. Unfortunately, my solution was to use Pipedream to be the middleman between the requests.
So it went 3rd Party Webhook Request 
Pipedream to send the message on Wix HTTP