I can confirm that I've also seen good CA availability on united.com when searching earlier this month, but the last few days it's all gone completely - and not just the routes I want, but in general, no CA flights show up anymore, as of a few days ago earlier this week.
Now, mind you, what I saw online I couldn't actually book online (on the old or new site - about two weeks ago): it would go right to the very last step after collecting CC info and just give a page titled "Error" that is completely blank. Tried different browsers, computers, CCs, but that didn't help. I called in and the agent couldn't ticket it either, saying that availability is not there anymore (phantom availability - she could also grab it and "confirm" it, but then it was "gone" when trying to ticket, she told me). Transferred me to web support and the person there saw the problem, but put me on hold and never came back. I suspect that each time I tried to book it, it grabbed a seat, but then united.bomb couldn't ticket it but the seat still remained taken still for a while. I say that as I called back a bit later and the phone agent could book it quickly and easily no problem (still was not bookable online then).
So, when/if you do see CA availability online, if it doesn't ticket, don't try again and again - you'll just use up the available inventory of seats without getting it ticketed (but not saving an unticketed PNR either, both me and the agents checked on that first). Simply call to get it ticketed then. If the first agent can't do it, HUACA. It's tricky, but can be done..
At this point, I take it, if one sees CA available on ANA or LifeMiles, just call UA to book it, right? I mean, UA is not doing CA blocking, is it? So if it's available on ANA and/or LifeMiles, UA should be able to see it and ticket it over phone, even when not showing up online, right?