I'm having trouble pricing/booking my UA mile run.

If I use UAL.COM, I get either a very high fare or it refuses to price it at all (especially the search for lowest fare option).

If I use Travelocity, I can't enter more than 4 segments.

If I use Expedia, it tells me there aren't any fares between my segments, offers another airline even though I've said only UA, or changes my routing for me.

I see a lot of 6 to 10 segment mileage runs being talked about. How are people getting prices and/or booking tickets?