I have a wild caught box turtle and it did not eat for 2 or 3 days after capture. Then, it started eating but only when not being watched. I use an outdoor enclosure so I had to stand back far enough that he could not see me. I would say try a fresh strawberry slice. They love them!
My male boxie Rex is now been with us a couple years and he will come right over to us for hand feeding. We also have a female boxie Jaz that will come over for hand feeding too. They eat a great variety of foods including: slugs, crickets, june bugs, earthworms, peaches, apple, cantalope, strawberry (favorite), grapes, carrots, snow peas, bell pepper, romaine lettuce, sweet potato. I'm sure there are more but those are the ones I can think of.
One trick we use to get them to eat a variety is to use the pulp from our juicer. It is a variety of fruits and veggies and the turtles really go for it. To us it is just using something that would go in the trash while to turtles it is gourmet food!
GOod luck with your boxie and keep us posted...