Testing leads stuck on 'pending'

Hello! I am working on an integration for leads with the ads I run on facebook and my CRM. For this I followed the steps on setting up an endpoint and configuring the webhooks product on the meta developer pages.
I managed to get a leadgen from the webhooks testing tool using the specific leadgen testing tool from the app's dashboard. I also checked and verified that my page-access token has the required rights to access and retrieve leads.
I however run into a problem trying to simulate the full lead ads testing tool (https://developers.facebook.com/tools/lead-ads-testing/) for simulating the entire process of making a lead and retrieving its data. My problem is that when I create a lead, the status of the test-lead stays on 'pending', and the Error Message displays 'Updated lead is still being processed and will be delivered shortly'. It seems as if the request is stuck.
I also verified that my app is indeed subscribed to the leadgen field using https://developers.facebook.com/docs/marketing-api/guides/lead-ads/quickstart/webhooks-integration/
I am using ngrok for tunneling the requests to my local environment, but that should not matter since I do receive the leadgen tests from the leadgen-specific testing tool.
Does anyone have any fix or idea what could be wrong?