Might not be the same issue for you, but I've never been able to book return flights to Asia with a stopover in HK online at BA.

For example:

LHR-TPE : no problem, and it's via HK.
LHR-HKG (stop over)-TPE : error, we don't fly HKG to TPE.

For the above I'll book via Expedia or another agency, but if it's ex-EU and hence needs 6 flights (excluding positioning) then I'll ring BA and they'll waive the fee once I explain, and convince them I'm telling the truth, that it can't be booked online.

I once managed to book an ex-EU using AA's website but it now doesn't even try with something so complex.