I often find that upgrades are not available on direct/ one stop international flights.

For example, UA # 1 used to fly LHR-IAD-LAX as part of its round-the-world route. Trying to upgrade was often hard, as the exact same seat number would be needed for both legs. The seat map might even show the cabin as sold out, because it will only show open seats for the entire flight number.

BUT, there are upgrades available for pax just flying LHR-IAD, or IAD-LAX.

So the trick is that you should ticket this flight as two separate CONNECTING flight.

Not only will you get the extra miles and segments, but you have better upgrade chances as well.