Henna does not generally work all that well on breasts/stomach because there aren’t any calluses there. The darkest stain will always be palms of hands and soles of feet. A chest design will have to be left much longer, or done twice to get dark. Of course, it depends on your skin, the quality of the henna, and how well the henna was made.
And by the way, henna in kits is not necessarily fresh. Try to get some from an Indian grocery store, or buy from [link deleted].