Give her some hides or move her to a smaller tub. Just because your other snakes might do fine in the setup they're in, doesn't mean she feels secure there. What kind of substrate is she on? Have you tried something "fluffier" like aspen? If your ambient temp is 84deg, what is your hot spot? That is a pretty warm ambient and doesn't give much gradient if you also have a hot spot in the tub. Have you tried leaving the F/T or a live rat pup in overnight without disturbing her?