When we got our doxie Spike as an adult, he moped around for a while, but finally warmed up to us. He also picked at his food for a while. He came from a good home, but my husbands uncle couldn't keep him. We have had him for a year now, and he is a very happy dog.
The treat thing is a good idea. I actually do that when mine get finicky. I usually give them liver treats or a tiny peice of boiled chicken.
Have you tried toys? Like balls, ropes, and squeakys? My baby Triss LOVES squeakys! Her favorite is this akc brand rabbit squeaky. Maybe get down on the floor and squeak a toy around her then hide it behind you? See if it gets her attention.