Booked JFK-SFO-SYD 6 weeks before departure during peak season and waitlisted for 30k miles upgrade to C. Seat maps consistently showed C cabin booked full on a 747. Missed out on JFK-SFO upgrade, and name was not called at gate SFO-SYD but hung out anyway. Got the last Biz seat at the very last second before boarding. I doubt they would have searched me out in E+ to escort me up front, they would have called another name had I boarded. Day before SYD-SFO return, my waitlist suddenly disappeared off my itin so I figured I got it. Got to SYD check in desk and was told upgrade did not clear but was still listed on res. I asked the agent to check and miraculously, the upgrade cleared and I got my C seat upon check in for both legs.
Just goes to show UA inventory management is as fluid and murky as one of those balls filled with liquid and snow that you shake, and I truly think that there are so many variables (missed connections, last minute cancels, holds that don't eventually get ticketed,.....) that you need to always keep trying, and take advantage of alliance searches as many times as you have patience for.