I am far from an expert on BPs...but since you specifically asked me to join the party -
What is the hottest you have had her temp range? And the coolest?
Have you tried a smaller box?
FWIW, I have an 05 girl that went off feed each of the last two winters, and has already started her foolishness this year.
I have also heard that a lot of people have that same complaint about their albinos (note - I did not say albinos, as a group, are poor feeders...just that I have heard many people express that sentiment about the ones that own them)
Unfortunately, with her in a rack with a group of others, it is more difficult to play the "see what makes this one happy" game. I think my first shot would be with a smaller box, narrower temp gradient (easier with a smaller box), and probably slightly higher temps. If that doesn't work, it might be worth dropping her temps a bit overall - pretty much just enough to let her metabolism slow a bit - let her play her winter games , then warm her back up and try to get her going again.
I'm sorry I didn't pay more attention to her the other night...I can't even comment on her appearance/body weight. If she IS looking thin to you, it might be worth trying to assist feed her...she might just like it