Try to book an award ticket from SIN to SGN (Saigon) via Hong Kong but .bomb doesn't allow me to do it. This seems to be a simple South Asia route.

SIN - HKG, UA 896 (Arrive HKG 1035am)
HKG - SGN, UA 869 (Departure HKG 805pm)

Anyone has any idea? Is it because of the long connection time? I can purchase this ticket online but the fare is something like $1600 .