Debugging
After you have setup your webhook, GitHub give you a nice way to replay events.
Retrigger Events
Navigate to your repository and the webhook you previously setup.
If you scroll down you should see a list of recent deliveries.
These are all events that have been generated and gone through your webhook.
If you navigate through the events you should see an event that was raised for your pull request. This is an example of what you are looking for.
If you hit the button Redeliver button this will re-trigger the event again and status-checks will run your functions again.