First, this is normal. My kids were both EXCELLENT eaters as babies. They would eat anything. At about this age, they started to get picky. Just keep introducing it and eat these things yourself in front of them. If they see you doing it, they will catch on. A few things I did during this stage: I would hide veggies and fruit in other things they liked (i.e. put baby food carrots in the spaghetti sauce, put baby food sweet potatoes in pancakes, etc.), I drink coffee in the morning. At this age, they like to mimick you. I would put Pediasure in a small, kid size, plastic coffee cup and tell my daughter to sit with me on the deck and lets have coffee and talk. She just thought it was so cool! She calls it her coffee and loves that stuff. It's packed with vitamins and minerals - good to fill in for whatever they might be lacking that day. It doesn't tast bad either: available in Vanilla, Strawberry and Chocolate. Good luck - it's just a phase!
A quick question--What is she drinking throughout the day? For if she is drinking a lot of milk or juice--she is fillingup on that and is not really hungry. For protein you could try scrambled eggs, crackers and cheese..just try to keep introducing her to new foods it may take a few tries until she will like something new.
If you can't get her to eat anything else then get her to take a multivitamin or (I think) Pediasure, that will help with the vitamins and other things she is missing from not eating. If she eats gummi bears she will eat one of these because they come in "gummi" forms.