You didn't mention what flight. With the exception (AA only?) of when an upgrade class is the exact same booking code (A in AA's case) as a discount-first class (on domestic 2-class planes), finding upgrades that will clear at the moment of booking can be very tough on any airline. Most people have to either "trust" or "risk" that they'll get miles upgrades if they go an upgrade waitlist. Often an airline will open upgrade availability only "momentarily", to the point that if you weren't waitlisted for the upgrade, you're not likely to ever see it.

Unfortunately, IIRC, United charges the co-pay (or requires booking higher-cost fares) for a miles upgrade at the time of booking, whether or not you end up getting upgraded. AA, by contrast, doesn't charge the co-pay unless/until your upgrade actually goes through, and allows upgrades from most any fare. So putting yourself on a miles+co-pay upgrade waitlist at AA is not quite as (financially) "dangerous" as doing it at UA (unless UA has recently changed their miles upgrade requirements and processing).