Porblems with lead/case form links and existing customers

Hi not sure if this is a really obvious mistake I have made but, created about a month ago a set of forms for enquiry's using lead form linking into a case form (to get around the stupid unsubscribe form email campaigns which we need for urgent product updates/firmware).

In the link was the information from the email, first name and last name fields so that the case form would automatically be populated and only the actual subject and enquiry needed to be filled in. And the duplicates rules would merge the two records.

This was working fine (running on two of our websites) until we noticed yesterday it was not working properly on one website, the email would be filled in on the case form but the first and last name fields were left blank.