Unfortunately that's one of the most commonly asked questions about GT4 that we've ever seen.
The answer is
always the same.
Quite simply you've missed something.
You need to check for certain that every gold trophy is in fact gold, it's quite easy to confuse gold & bronze colour on some monitor displays.
Also you need to make sure that you've completed the championships, not just individual races.
There are 3 events to check in Professional Hall:
Tuning Car Grand Prix, World Classic Car Series and World Compact Car Race.
Make sure you have a championship gold ahead of the 5 gold trophies for the races in each of these events.
If you're having trouble you'll have to download a race list and prize car list.
Check you've got all of the prize cars that are associated with each of the events in Beginner & Professional Halls. (If you've sold cars, you'll have to go through the Diary in your Home screen to check that you won these.)
Also you can use the Diary to check off the races one by one to ensure that each one has the winning stamp next to it.
It may take a while but it's the only way to be certain, however, I guarantee you'll find that you've missed at least one race somewhere, or got a bronze in place of a gold.
If you have a MAX drive, you could also copy your gamesave and post it up here and have someone take a look at it for you.
Also, as an aside, capital letters are expected here at the beginning of every sentence you type, as is a capital "I" when you refer to yourself. "I've" has an apostrophe too. Please don't be lazy when typing here, it's one of the rules in our
AUP that you agreed to when you opened your GT Planet account. Thanks!