Sprout Forms 3.2.0 added an Integrations API and the previously named 'Payload Forwarding' is now handled via the Custom Endpoint Integration.
The new Integrations feature also improves tracking of Integration submissions so you should have more data to help monitor or troubleshoot if any requests are behaving as expected.
Every API you submit data to is different and Payload Forwarding may need to be implemented differently depending on what service you are trying to submit to.
By default, the data that gets forwarded is in the same format that it would be in if you were submitting it to Craft. Many services may require data to be in a different format, so we'd need to know more about where you are trying to submit the data and how to help troubleshoot further.
See a brief overview of Payload Forwarding in the docs. If you need to submit the data in another format, you could consider overriding how the fields are structured in the templates with Template Overrides.
We do have on our list to refactor the Payload Forwarding API to make customizing the payload for different services more accessible.