this exact thing happened to me today, res was hung up by Concur through United website booking

This would be my guess. An unticketed PNR is not eligible for upgrade.

I've had similar strange things happen. Recently I was not on the upgrade list but the BP showed 'added to upgrade list." Asked the gate agent who didn't know what to do. Was a moot point anyway as F was full.

Indeed.

But the 1K desk specifically said it was ticketed and couldn't understand why the error ... they reissued the ticket and that fixed it.

"This reservation is not eligible to be upgraded."
 

I have 2 trips booked this year, and when trying to upgrade the fare (intent is to use my PlusPoints), for whatever reason both of them say the error "This reservation is not eligible to be upgraded."

These are not awards flights, I'm paying money. One has fare classes K and Q. The other has fare class S.

I remember calling in the 1K Desk to ask if they can apply PlusPoints or do Miles+Money they said they could but could not answer why I can't do it on my end. They told me to try to wait 24 hours, since I had just processed the reservation, but by now it's been weeks.

I had the same problem with two itineraries in the past 10 days. Both Govt award tix S class of the type I have upgraded many times in the past. It's a pain to call with the low quality of "1k" agents these days I fear doing so will mess up the itinerary. I had one decide to pull miles when I had asked to use PlusPoints. That's why being able to do it myself is important to me.

Are these intl and basic eco? That is the only type that is not upgradable

Not basic. I never buy basic since we want seat assignments and carry on bags.

One is international, the other is not.

With kids these days it's hard for me to do as much work during the day, so I called at night and my wife heard me on the line with clearly second shift 1K voice person and shook her head as she could hear how useless the support agent was. I didn't push too hard on it and the support was abysmal. However, when I do manage to get some free time during the work day, when I call the 1K line it's usually been pretty good. In my experience it's the experienced ladies who clearly have 30+ years in their voice who do the most, but those tend to be during the day only.

I wonder if the issue is because I have a lap infant. This is the only thing I can think of that is different from every other flight I have been able to apply Plus Points on.

Very likely that's the issue.

Lap infant always requires agent to apply upgrade. For international, they are also supposed to collect 10% of the fare.

Yeah, that's unfortunate. Sometimes I just want to look at my options like how many PP does it take, if i want to pay miles how much does that take, etc.

That is pretty straightforward
https://cimg7.ibsrv.net/gimg/www.fly...0385847496.png
The fare breakpoints between 40 or 80 PlusPoints is W.
If multiple segments, the multiple segments are all covered by the single highest requirement

For mileage upgrades, see https://www.united.com/en/us/upgrade-awards

That’s true, the PP rules are well published. But what about Money + Miles?

See the link in second part of post (https://www.united.com/en/us/upgrade-awards - requires querying specific route).