Sounds like a lot of bad luck on highly competitive flights. I understand that among 1Ks, Million Mile 1Ks are at the top of the list, and then it goes by fare paid, unless you are within the timeframe close to the flight where things are handled by the Departure Manifest System (DMS). In the DMS system, things like time of checkin start factoring in. There have been some recent reports about the DMS priority being changed. I don't think that debate was settled decisively. I'm not a DMS expert. I'll let someone else speak to that. But it doesn't really answer your question other than this: If you haven't cleared on the day of departure, showing up early may be the only way to give yourself an edge.

What else can you do? Other than flying a few hundred thousand miles for Million Miler status you can try strategically selecting your flights to be more likely to have open F/C seats (midweek, mid-day). Fortunately for United, there are many flights which almost sell out the F and C seats, leaving very few upgradeable seats. You may be trying to upgrade on these.

Watch out for which flights actually have any kind of service. I've found that when there is no meal service on a 1.5 hour flight in the mid afternoon, the upgrade space is always there. Lunch and dinner flights seem a bit harder to clear, at least on the routes I fly. If a meal is a big thing to you, that may be a minor factor in your upgrade success.

For international flights be sure you are on the PA list. It carries higher priority. I don't think there is any equivalent list for domestic flights.

I just re-read your post, so I'm adding this: In my opinion, you are making a huge, collosal, enormous mistake by trying to upgrade using the website. United's website is utterly buggy. Unreliable in the extreme. The seat assignments that appear on line are totally different from what reservationists can see. Always call reservations. Politely hang up if the person doesn't sound competent and try again. Time and again I've found that I can get information about how many seats are left to sell, what seats are unassigned, if there is a meal service, or more important, discover that a schedule change has wiped out my request for an upgrade. Reservations can't tell you a few things that friendly gate agents can; such as where you are on the list and how many of the F or C class have checked in and that kind of thing.

I find calling reservations is comparatively quick, at least the 1K line is. I just don't believe the marketing that United is saving me time by using their website. Since their website is disfunctional on what matters most to them: presenting United flights for sale at competitive prices, why would I believe what the screen is telling me?