If it's just food/veggie reluctance in the face of tastier meat and processed food, then your turtle sounds like mine (and most kids!). I feed in a cycle: Stage 1: some crawlers in the basin, along with occasional crickets and other bugs, then Stage 2: brightly colored fruits/veggies (she's getting strawberries Sunday, probably cantaloupe too), and now and then Wardleys (my girl barely sniffed at Reptomin, it's interesting to hear that corn might be why--she loves whole kernel corn too, which often makes its way into Stage 2 along with bright green broccoli, etc).
I've read alot that emphasizes box turtles are visual and are more likely to take fruit and veggies that are brightly colored. This has increased her f/v consumption, alhtough as a 3toe she is primarily a carnivore and I really just use f/v to make sure she gets variety. The trick for me is that I don't return the feeding cycle to Stage I until she's at least tried whatever fruit and veggie that's in there. After a couple of days of refusal I go to Wardleys, and if its clear she won't eat whatever I have offered I toss it (after 12 long years of this I have a pretty clear idea of what she likes and dislikes, although she has thrown me curveballs too). But usually she will eat Stage 2.
A couple of additional discoveries: 1) Refrigerator-cold fruits and veggies get rejected, so I let it get to room temperature BEFORE I give it to her. She'll get some broccoli Sunday too if I remember to deforst it in time. I don't know if turtles can smell a refirgerator odor on food or what. 2) Maybe in keeping with this, I've found she tends to be more amenable to seasonal eating. I don't know what kind of internal clock box turtles have, especially since my girl is a pet shop turtle who has never hibernated in her life, but she seems to like stuff in season even when it was bought at a big city supermarket and not a farmer's stand. She loves dandelions too but I actually need to buy those in the store because of all the pesticides used.
If your turtle will eat a quality box turtle chow you probably have nothing to worry about, because there are plenty of fruits and veggies in there. My problems were that my girl was rejecting turtle chow AND most fruits and veggies, and I knew she needed more than crawlers and crickets to survive.