My personal opinion is that of the girls left, Jessie, at her best, has the most "star quality." She is also the one with most weaknesses. At the beginning of the show, Jess had a spark that meant that I didn't notice this weaknesses as much or I could gloss over them. However, as the weeks have progressed, Jessie's confidence has faltered, and she has lost her spark, which means that I notice her weaknesses all the more. And to be honest, despite her good vocal ability and star quality, I can completely understand why she would have been rejected from drama school as she is weak in both acting and dancing/performing.
I'm not sure if Jessie weaknesses can be ironed out. However, unless she gets some clear direction, we're never going to know. They really do need someone on the panel who has experience at coaching performers, and so can not only tell the girls what they could improve on, but also how to go about doing it.
At the moment, she is probably the most unreliable girl. At her best, she could be amazing, but similarly she is also the girl most likely to fall flat on her face if she can't work on her weakenesses.