I have the Same my big toe nail missing. I use a form to mark out the size of the nail, then sculpt on the form (avoid the risk of over exposure reactions by applying it direct to the skin) and then use adhesive to attach to my toe once it's 'cured'.
mine last about a week but my mother has the same answers only lasts a couple of days, I can only assume the skin produces natural oils which is why products lift when they contact with skin, hence why it does the same? I'm sure someone will confirm this or correct me if I am wrong.
Hope that helps xx