I have only faced this issue a couple of times. I'm Silver. My wife rarely flies. I booked us on round trips. I wasn't expecting an upgrade. I could move both of us into E+ at T-24 on the outbound but could not move her on the return even though there were many E+ seats available.
Since, both times, she got E+ on the outbound, perhaps for future flights I should just book two one-ways??