Check you are using the correct Webhook URL:
Go into the PeopleHR integration site.
Log in or register with a new account with your admin email address.
Click on PeopleHR Settings tab.
Paste your API key in the box provided, and click Authorise.
To get the Webhook URL, click on Here.
You will be directed to a blank webpage - highlight and copy the URL at the top of your web-bar.
Now return to the API section in PeopleHR and paste the URL into the Webhook URL section.
If you have created a Webhook externally and configured this for JSON, try changing this from JSON to application/x-www-form-urlencoded and trigger a new event to see if this is received.