The first thing to try: When you type in a destination airport (or origin airport, if needed), type in part of the name and then wait for the full entry to load as an option (eg "Hong Kong, Hong Kong (HKG), Hong Kong" rather than "hkg" or even "hong kong") before selecting it.

That cures some of these problems, caused by the webpage not recognising whether you're searching on a long-haul or short-haul route until that step is taken.