How can I integrate my form with Zoho CRM when the crm fields don't appear?

I am trying to add an additional field into my form and update the integration with Zoho CRM. 

I have created a new custom field within leads in the CRM and a new question in the form to match this. However when I go to edit the integration the new crm field does not appear on the list, so I cannot select it to integrate with.

This is the first time I have used a custom field rather than the CRM standard ones, and I'm not sure if it impacts this. What can I do to get it to appear on the list?

Thank you for your help.

It is possible that the custom field you have added is something that the field mapping on the integration wizard could not understand/recognize or has no means to integrate. Can you try adding a single line text field if it is possible to map on the integrations wizard?

Thanks.