You must find out whether it is a human, mapping, or connection problem as they each require a different approach.
Troubleshooting problems with custom APIs with HubSpot can be a tricky process to nail down.
Firstly, you need to define whether the issue is human error, a mapping problem or a connection problem.
If data is not pulling through into the fields correctly, it’s always worth double checking the values entered are legible. For example, putting letters into a number only field would cause an integration mapping error, the same goes for special characters.
Once you have checked the data is valid, you will need to see if the problem is due to a mapping issue. Are the fields that need to be transferring data the same property type? For example, a single line text field in HubSpot may not map correctly to a number field in your other system. Always check the field types are aligned.
Next, if you are using an API key for your custom integration, you can head over to Settings > Integrations > API key > Call log to see if there are any connection issues. If you are not using an ApI key, rather OAuth, it’s always worth checking your external logs for both HubSpot and your other system.
If you are experiencing any further issues:
The first point of call is to check the developers forum to see if your question has been answered before: https://community.hubspot.com/t5/HubSpot-Developers/ct-p/developers
This forum is a really great place to get advice with troubleshooting HubSpot integration issues. If you do not find any relevant information to answer your question, post one of your own tickets and the developers are usually quick to respond.