There are actually several things you can try. Have you tried different brands of hay? Or different cut of hay? Although all timothy hay is pretty similar in nutrients different brands seem to make a difference in taste.
I know it's probably obvious, but how does the hay look and smell? It should smell nice and sweet, not moldy/musty or dusty. It should also not be overly dried out and brittle, it should be a nice green color not brown or grayish.
Another thing you can try is offering different kinds of hay. Not only can it help keep the hay interesting but different kinds of hay are chewed differently, which helps keep the teeth wearing down evenly. Try mixing a couple different kinds of hay together to give more variety. Alfalfa hay for example is more flavorful and can interest chins to eat more, however if you already feed alfalfa based pellets keep the alfalfa hay to just a little and the rest timothy or other grass hay. I normally have 2 or 3 different grass hays for my guys at any given time, as well as alfalfa hay cubes. Hay cubes are another thing you can try, I prefer to offer them in addition to the loose hay, but they can be something different for the chin.
The last idea I've got is making or buying, something like
Fuzzies Kingdom Foraging Hay mix. It tends to sell out quickly, but you can get an idea, put chin safe herbal treats into the hay to add flavor and create interest. There are a lot of different plants on that website to give you an idea of what you can use.