It seems that the inbuilt Search feature isn't that reliable as yet.
We'll look into that.
In the meantime if you don't get the search results you'd expect, you can try Google's site: method.
i.e. To search for Biggles in the site you'd put in your Google Search Bar:
site:fuerteventura-forum.com biggles
This will return results from both the old and new versions.
To narrow down the results to just the new forum add "forum.com/index.php?" - including the quotes i.e.
site:fuerteventura-forum.com "forum.com/index.php?" biggles
To just search the old forum use add "forum.com/topic.asp?" i.e.
site:fuerteventura-forum.com "forum.com/topic.asp?" biggles