When you're feeding an aquatic turtle, it's important to remember that they generally, always eat in the water. If you're offering them food outside of the water, they may not eat it. When you're feeding a land turtle, it's important that you feed it in a bowl, or on a piece of rock, or something that will keep the substrate that's lining your enclosure from sticking to the food.
Some of these turtles and tortoises will actually accidentally swallow parts of their bedding material. I have actually seen a case where a turtle was fed its meal in a thin Styrofoam bowl, and it actually ended up consuming some of the Styrofoam. That made it very sick.
Another problem that we see sometimes in our tortoises are overgrown beaks. So, again, some people used to think this was related to vitamin and mineral deficiencies in their diet causing the overgrowth of the beak.
There are some cases now that show that if you feed your turtle on a slab of rock or something more abrasive, as they take bites of food, it will help to keep the beak worn down. In their natural environment, they probably would be doing a lot of foraging and their beak would wear down naturally.