Hi Tracy, based on Jira's spec, you should be able to get a message out of the 400 response with more detail. In their example, the 400 response comes with JSON describing the issue like this:
{
"errorMessages": [
"Field 'priority' is required"
],
"errors": {}
}
You should be able to capture this information by adding custom error handling (custom with rollback) to the REST call step. In the error flow, you'll have a variable available called $latestHttpResponse (you have to enable this checkbox in the REST call activity settings). You could then log the value of $latestHttpResponse somewhere or use the debugger to capture its value.
Tracy,
One thing that I do see in your post is that the Json comes back with a 200 status code. Have you taken a look at the service as it may be having a problem after you do the POST?