Data is not being passed from web form to Zoho Leads screen

Data is not being passed from web form to Zoho Leads screen

Hi all

I have a custom form on my website (not a Zoho 'webform'). When a user submits the form; form data (customer name, email etc...) should obviously be passed to my Leads screen. I have this working BUT - for the life of me - cant get all of the fields in my Leads screen populated!!

I cant think why this is. (Ive tried many different things). I dont understand why some fields get populated and others dont. 

The form submits to some php script which includes the following code (Ive hardcoded the values for now):

$post_data['PostCode'] = 'PL161BY;
$post_data['Test'] = 'A test string';

I cant work out why PostCode gets populated on my Leads screen, but Test does not.

Can anyone shed any light on why this might be? As far as I can see all my variables in my code marry up to the field names on the Lead screen.
Thanks