If you change your formation you must select the Succubus & Xyless for the party members. It was probably meant to require them to be selected at the start since they are the ones with you, but it wasn't added to the program. (Like you are required to select Carys when you help her after reading the note she left)
You also must have Ruksana as a member of your active party. If she is, try removing her while you are in the are where the scene takes place, then move south to the next screen, the add her back into the party and go back.
I have never had any problem with the scene activating. If you still can't get it to work let me know and I can take a look at your save file if you upload it.