Manduca sexta will eat all kinds of solanaceous plant material, but once they've eaten one that they prefer, they might not accept others. Are they wandering off the plant or are they just not feeding on it? If they are wandering off the plant and are late fifth instars, they may be looking for a place to pupate. If they aren't fifth instars and aren't wandering off the plant and are just not eating, they may be preparing to molt into another instar.
The lab I work in keeps Manduca sexta and they're kept on an artificial diet that they'll accept as long as they're not provided with actual solanaceous leaves beforehand.
I've reared them on tobacco, tomato, Datura, Solanum laciniatum, Solanum nigrum, and Solanum rantonnetii.