Originally Posted by
JPDM
Your question is not clear. You are traveling to where and when?
If there are tickets available on 12306.cn then the agent can buy them for you.
The fact that there are tickets for the original departure city of this train to the final destination does not mean that there will be tickets for intermediate routes. The seats (beds) are allocated to the various segments and there are usually much fewer tikets made available to journeys between intermediate cities.
Back in my train travel days, I used to run into the OP's problem often (sleepers from many station stops are simply NEVER on offer).
The standard --usually, but not always, reliable -- way to play it is:
1. Buy hard seat
2. Position yourself on the platform in front of the 补票 booth (which is on the train that has yet to arrive, but its location is a known variable to train station staff)
3. Upon boarding, state your intention to the person seated at this booth
4. They can often issue new tickets on the spot as soon as you hand over cash for the fare difference, but sometimes you need to wait (e.g. once, on a 15 hour train from Nanjing to Beijing, I didn't get summoned for the upgrade until 6 hours into the journey; I took it in a heartbeat!)
#2 is the key step in this strategy wrt maximizing your odds.