If you search around, you'll find a few threads here about ITA with some know issues. Among them are:

-They don't always get webfares.

-Their inventory system is not real-time. If you look at the fine print on results page it says what time and date the fare they are quoting was loaded into your system. Ususally within a few hours, but not a direct look at that second's inventory.

There are other things that might be affecting your search too. Are you changing the purchase city? I know I tended to breeze past that but it might make a difference as well. Might there be some restrictions on which airlines/GDS can sell tix originating in China?

I agree that it's not perfect. (If there really was one site that was perfect, it would blow the rest out of the water.) But IMO it is very good and a great tool for searching with a powerful search language that really allows you to drill down on what you want for complex itineraries.