Just keep trying. Ragnar likes to crack open almond shells and get the almond out. Then we take the almond shell and put a little pureed baby veggies in there and a thin layer of almond butter. He will sometimes eat the food out of the shell.
I also tried making veggies a hanging toy in his cage. Hands down though.. he will ALWAYS eat any fruit or veggies that is freeze dried. Light and crispy. For him it must be a texture thing? He would definitely like to eat only pizza and burgers and fries too. Just keep trying. Different sizes, shapes, hard, soft .. you'll stumble on it eventually. As long as he is eating pellets and seeds he wont starve. If it persists you might want to consult your vet. I am certainly not an expert, but I did get some awesome advice here.