We have a custom search form enabled using the custom fields addon. At the moment fields are - display name, first name, last name, town or city. Searching by display name or town works fine but searching by first name or last name returns no results. In wpforo settings I have the search function set to "filter".
Does anybody have any ideas why this might not be working?
Your question is related to wpForo addons and not to wpForo itself, we provide here only wpForo main plugin support, the addons have their own forum at Gvectors.com
About your question, For now you can create new First Name and Last Name fields, and insert them in the Members Search Tab.
I have also notified Gvectors about those Fields, so they would insert them in the Members Search Tab.
For further question about wpForo addons please ask them at Official wpForo addons forum