Basically, since the new "enhanced" system started the GuestSeat availability as been drastically reduced (I think 1-2 seats per flight in business on a 787). In the past they used to release extra GuestSeats 1 week before the flight if there were spare seats. That is no longer the case.

So if the limited availability is booked then you will not be able to upgrade with miles unless someone cancels that GuestSeat booking. The inflated upgrade with miles prices you are seeing are in fact a paid upgrade then converted to miles at something like 0.07 cents per mile.

ADCB upgrade vouchers only work if there is a GuestSeat available in the higher class.

I use seatspy.com to monitor the GuestSeat availability, you have to book really early to get one nowadays. And sometimes they don't even appear, as i suspect on certain routes they block seats in advance for priority clients before they release them.