How to get dark stain on back?

One of my customers has had me henna her whole back twice...once a lovely giant lotus and second time beautiful peacock feathers. I told her to dry it, spray seal it and suggested wrapping in tissue or even using press an seal around herself. SHe used the spray but wanted to brush it off prior to sleeping (did not want any little bits falling off and resticking somewhere undesired !) Anyway, the stain was not dark and she is disappointed.
Is the back going to stain lighter just like an upper arm or belly? Is there anything I have her do to darken it? Will sleeping with it really help it?
My henna works great on hands...feet....so I know its strong. Thanks

As far as I know, the farthest from toes and fingers, the lighter the stain. She shouldn't have brushed it off, she needs to seal it, wrap it, and sleep with henna on to get the darkest stain possible.