In Jan and Feb I flew BKK-NRT-IAH with a ticket purchased from CO for 1200USD. Both times I was able to upgrade NRT-BKK using a SWU. I just purchased the exact same route for the same price, but now UA is saying SWU not available. I am flying CO metal from NRT-IAH and UA metal BKK-NRT. Can anyone shed some light as to why a SWU is no longer an option.

Thanks