I would try some sweet smelling fruits and vegetables like a small piece of banana or strawberry and even sweet potato and gradually mixing in greens so that they can get their vegetables. However be careful to let them not eat too much fruit as it can give them diarrhea. Are they young box turtles? Younger box turtles seem to be more carnivorous than omnivorous.
Just keep offering them fruit and veggies. I usually put a dish down and top it with worms, could take some time and some won't eat any or very little especially some of the older older ones. Let us know how it goes.
I start my babies out like Millerlite said...I chop up some healthy fruits and veggies and mound it on the feeding tile, then I place the worms or pin head crickets or meal worms, etc. on top of the pile. They will accidentally get a mouthful of the fruits and veggies while trying to capture a worm.
Congrats on the banana success. Keep offering fruit and variety don't get stuck on just bananas. Also if he likes bananas but it on top of lettuce and he might end up biting into it and eatin that as well. Usually from what I've seen from boxies they are scared to try new things once they do tho will notice and eat everything. Box turtles are picky tho. I have some that don't like banana, or strawberries some love them just gotta keep trying everything.
Try different greens. Kale, mustard greens or dandelions (leaf and flower). Kiwi seems to be a common fruit that a lot of boxies enjoy. Mine goes nuts for the stuff. And blueberries too. He'll smell them even when he is fully buried.