My dog won't eat

I have a Yorkie, Tinkerbell. She's about 16 months old. I'm having a bit of a problem getting her to eat by herself and eat just in general. She gets fed in the morning, but she won't eat. She then gets fed in the evening, pellets and boiled chicken, but she will only eat if I stand there and she will only eat the chicken. We have tried so many different pellets but she will not eat any other type of pellet unless our other two dogs eat the same thing. She lost a bit of weight and I'm starting to get a bit worried. How should I handle this?