I plan to buy 2 economy tickets from Newark to Edinburgh on United and then use miles to upgrade them to BUS. But every single date I check has "no award upgrades available". I am more than six months out. What gives? How can every single day be unavailable for upgrade unless maybe they don't release the allocated seats until a certain date???
Most airlines limit the number of seats on each flight for upgrades.
Sometimes there's a fixed number from the outset, but otherwise they may release them in bits - a few at a time.
If it turns out tickets for a date are really popular, it's better for them to sell pricey tickets, and offer upgrades on the quieter days.
So some more may come up if you check regularly, but you may also have picked a really busy week - an event on (Edinburgh Fringe Festival? The Tattoo?) that the seats are booked for already, or are more limited than normal.