Welcome to GTPlanet.
First thing you've done wrong is to not try the Search function before posting a new thread. I think this is in the sticky FAQs above, but there have definitely been numerous threads on this very topic. Try putting "world championship" in the search box, and limit it to thread titles on the GT4 board.
However, that being said, I'll answer up anyway.
You need to win all of the series races as a series, not just as individual events. If you skipped or placed other than first in any race of a series, you must go back and gold that race individually, even if you won the series overall on points. You need to make sure you've done this for every event in Beginner and Professional.
The gold and bronze trophies can look alike. Odds are that you either have a bronze you don't see, or you have an event you didn't run as a series.
Either that, or you don't meet the license requirements.