If they are losing weight, a vet visit is always a good idea. Just make sure that this vet is qualified to do turtle work. Just because they will, doesn't mean they should. Ask details about their experience with herps.
Are your turtles young? It can be very difficult to get young box turtles of all species to eat anything but live food. Try the prepared "box turtle" foods being sold by zoomed and others. Also, try soaking some aquatic turtle food in water and offering it. Offer every type of colorful fruit and veggie you can...tomatoes, cantaloupe, strawberries, grapes, prickly pear cactus, etc.
In the meantime, offer a large variety of living prey; crickets and grasshoppers with the back legs removed for easy catching, superworms, waxworms, mealworms, silkworms, butterworms, slugs, snails, and even pinkies. Dust them with vitamin/mineral powder at least once weekly, depending on how much they consume. You can roll earthworms in vitamin/mineral powder and try feeding them to the turtles. Superworms are very active and my ornates really love them. They're pretty high in fat for a feeder insect, so that may help adding some weight. Good luck!! Also, make sure that your other husbandry issues are in order. Most all problems manifest in one common way...the turtle stops eating.
