ISTFlyer, thanks for thinking about this . Let me be more explicit about my sequence:
1) Search united.com and find an itinerary that meets my needs, all in X.
2) Write to LAX sales office by email.
3) LAX sales office called me to call call center, 800-874-8875 , so I did -- and was told the SFO-ORD sector has no X inventory.
4) Recheck the itinerary on united.com and confirm X still available.
5) On a whim, see if I can price this on the M&S Star Alliance redemption tool, which worked only as to first segment but not second.
6) Write to Boston sales office by email.
7) They also replied to say the SFO-ORD sector has no X inventory.
What to make of that? Agent error is a perfectly believable explanation, but interesting that two agents (call center and Boston office) said the same thing, and web tool said that too. Of course could be all three are using the same flawed process which is not booking this multi-segment journey correctly. Hence my hope that an expert here can guide me towards the right statements to agents to help them do this properly.