I've been trying to do a 4 segment one-way award ticket from PVG to BKK and when calling United the agent is not able to book Singapore airlines flights.

My trip PVG-ICN-HKG-SIN-BKK will not price out at 22.5k in business, but PVG-ICN-HKG-BKK-SIN will price out to 22.5k. I asked the agent on the phone to just book PVG-SIN with a direct flight with Singapore and the computer will not allow that. When I try and book my original itinerary it bumps the price from 22.5k to 40k. I've called and hung up 4 times now. Any ideas? I'm within the 3 connections/4 segments on a one-way. United's website even prices out PVG-ICN-HKG-BKK-SIN as 22.5k. I know Singapore Airlines cannot be booked online starting last month.