Whata are the temperatures in your enclosure? Try to keep the temperatures at around 75°F inside and provide a basking spot with around 85-90°F with heat coming from a heat lamp or ceramic heat emittor. Do not use bottom heating elements.
If your indoor temperatures are lower, the turtle may think it is time to hibernate and will stop eating and get weaker over time. Also make sure to soak them in warm water every day, ideally in the morning to get their system warmed up.
More details about the temperatues and hibernation guides can be found here: https://boxturtlesite.info/light-temperature-humidity-for-box-turtles/
Temperatures for box turtles