I thought that any united/lufthansa flight to europe could be upgraded with milage plus mileage (assuming that there is a seat open of course) if I pay a co-pay with the miles.

However, when I search on United, there are plenty of United/Lufthansa flights to Frankfurt (I am looking at IAD-FRA), but they won't come up when I search for upgrade eligible flights.

Am I missing something? Sorry if this is a stupid question, I searched around and it seems the rules are pretty clear, and I don't find anything saying why these flights are not eligible for upgrade if I pay a copay.