Does your itinerary involve an international stopover? I have had this happen to me (or at least, had ua.com issue a "could not price" error) on itineraries with stopovers. The explanation I received from a res. agent was that (some?) international stopovers do not "auto-price" -- the rate desk has to manually verify the pricing.

My speculation is that the autoprice system doesn't know how to apply the "highest intermediate point" (HIP) rule, but that's mostly just speculation. To take an example, recently I was trying to buy a ticket SFO-FRA-PRG (prague), returning through ORD, and wanted stopovers in FRA and ORD. No can do online. There's a ~$2500 B fare SFO-PRG (BE1E) that allows stopovers (1 free, 1 for $50), but if you try to book this fare with a stopover in FRA, you get hit by the evil HIP rule and the fare shoots up to $3500 or so. Basically, PRG is a cheap market and FRA is an expensive one; if you stopover in FRA, HIP dictates that your fare can't be any less than the SFO-FRA fare.

Maybe one of our resident travel agents or ticketing gurus can supply more info.

As a practical matter, if it's a complicated itin I would simply call UA and ticket over the phone. The $15 you pay is pretty inconsequential in comparison to the pain of trying to do some of this stuff online. Good luck.