The problems you describe may be related to husbandry, i.e.: is the tank large enough?, are you using the correct food? Is the habitat warm enough? Is there a possibility that your turtle has parasites?
Many of your questions can be answered by viewing the following web sites:
http://www.redearslider.com/entice_eating.html
http://reslider.free.fr/backup/feeding.html
Please also perform a google search for more information.
If you can't find answers to your questions, please post again with detailed information. Thanks
So many questions for you: how long have you had her? What is her cage set up like? Does she have a basking log to get out of the water at times? Do you have heat for her to keep the ambient temperature up to 85 during the day? What about a full spectrum light (not an aquarium or grow light)? What are you feeding her?
Young Sliders are more carnivorous than adults so she needs more meat products like mealworms, bits of fish, etc.Crickets are sometimes used but they have a low calcium/phosphorus ratio and need to be supplemented.
As for plant material, there are some safe foods you can use and there's a long list of things that are toxic. It's easier to find out what you're doing now than put the long list here.
Turtles often have parasites and that can be checked by your vet. If the temp is too cold for her, she'll be sluggish and not eat. Some more information from you would be helpful.