Sounds to me like you're in a bit a bind. Even if the ticket is on TG ticket stock (issued ex-BKK) that's kind of not too relevant to the current situation (well I believe anyway - I could always be wrong!). As others have said, UA cannot see your amadeus reservation, so giving them that PNR is useless (and it gets confusing since they're all 6 character strings). My experience is that the issuing agent (in this case, your T/A in BKK) setup the "master" PNR in Amadeus, and when that is set up, SSR's are sent to the other airlines in the itin to setup records in their own system (if they don't use amadeus), as such each airline involved will have a different PNR identifier for the same trip - usually I ask the agent I am working with to give me those PNR's when they give the main one, or I grab it from another source (such as viewing "My Itinearies" on united.com). This is not crucial, since you can just give a flight number/date and they can locate your record that way.

So, if the T/A made a change to your flights, it's their responsibility to see that the other carriers get the info updated. However, if you change via one of the airlines one of their flights (eg: UA) who will them modify their "mini PNR", that will not be reflected back in the "master" PNR (for some reason the updates don't always get transmitted back, or they might send an OSI (note) to the T/A to let them know, as they would with a time change or other event like that - then again it is up to the T/A to notice it and do something (or nothing) about it).

The usual situation that this becomes important in my experience is when you do things like apply upgrades, since (at least with UA) they like to "take control" of the reservation to apply the upgrades, and then it's out of the hands of the T/A totally - so I usually warn the agent that this will happen - this is after ticketing, so the agents are usually happy to be rid of it. LOL. That's when I do all changes via united.

In your situation though, it seems like someone at UA SIN stuffed up somehow, and it needs to be fixed. Good luck!