Okay, here's a new one (as far as I can tell by searching this thread... might have missed something if it didn't have the key words I searched for).

I just had IAD-DEN-XXX booked (yesterday) in full Y. The reservation on united.com shows the "Premier Instant Upgrade Fare" logo with the yellow U. I clicked through on the "Instant Premier Upgrade" logo below, and it gave me the option to select both segments for Y-up. And, indeed, the DEN-XXX segment is now in JN...

...but the IAD-DEN segment won't stick, despite JN=3 availability. I've tried about 8 times, and while it initially claims to have upgraded the segment to JN, the yellow U logo is still there, as is my E+ seat assignment... and upon going back to the initial reservation, I see that I'm still in Y.

I tried the Premier phone line; two separate agents (and their respective supervisors) weren't able to do anything. They agreed that the JN availability was there, the upgrade should go through, but the computer refused to process it for absolutely no reason that they could identify. Nice people, and they were clearly frustrated at being helpless.

In the olden days, when the computer clearly screwed something up, supervisors generally had a workaround. Seems that's been locked down at this stage -- if "Computer says No!" then there's no way around it.

Update: I PMed UA Insider, and they fixed it within 60 minutes. You guys are awesome! Please please share your mojo with the rest of UA's team... (btw, no insights yet as to what was wrong or how it got fixed, so YMMV in the future...)