I got the Cash-Upgrade system messed up on a few bookings in the last years. Especially Multi-Segment flights seem to be vulnerable. Sometimes the hotline can sort it out and book the upgrade, sometimes it cannot be solved. Most of the Call-Center Agents have no clue about the system and the rules (you can only take one piece of hand luggage, even in business, you will still earn economy miles,...). Sometimes HUCA helps. Sometimes it shows availability on two segments, but they cannot be upgraded. If you select one segment, you can upgrade that, but then this will mess up the booking making the 2nd one not upgradable.

When I have Multi-Segment-Flights I try to put a bid on the upgrade auction first, as this link sometimes disappears after upgrading few segments. Then I try to upgrade online.

And then again: Even if upgrade your online BP still shows "Economy" which leads sometimes to discussions at the fast-track or the handluggage-police.