I did notice (after I got it working) that when I used an input that required a relationship Zapier didn’t receive the webhook at all.
So I could have inputs like Name and Email, which are just simple text and it worked. But when I added an input for a relationship (like another user), it wouldn’t work at all. It may be that using a relationship messes up the formatting of the JSON.
My goal was to use this form to immediately add a new user. Since the above issue makes this impossible, I’m instead using a form to add a record in another collection, and using THAT as the trigger to add a new user.
Downside is that there is up to a 2 minute delay between adding the user and the user showing up in the collection.
I hope that helps troubleshoot your issue.