Cockatiels usually dislike fruit. Mine does. She only likes dehydrated apple and strawberry. That’s okay because, fruit has sugar and is not needed. Vegetables like kale, brocolli, dandelion, boy choy, green sweet peppers, orange foods have vitamin a like carrots, cooked sweet potato, raw orange peppers. cooked green beans are fantastic foods for them. If here isn’t much produce diversity , at least offer them brocolli , kale and carrot. Kale is amazing and the best source of calcium, along with boy choy. You can also attempt sprouting , millet, kamut and sunflower sprouts have a wide range of vitamins with low fat content. Vitamin b,d,a,c can be found in sprouts along with protein. It will take time to transition a scared bird, so work on that first and once your bird finally eats a new diet, it will appreciate treats like millet spray much more, and become more brave.
Also, transitioning to a new diet can take a long time. All too often people feed their parrots only seed, which isn’t a healthy diet. As a bird gets older, their natural instinct is to not trust new foods after a certain age, to protect them from eating poisonous or inedible materials in the wild. So , you will have their wild instinct working against you. I suggest chopping vegetables up very tiny and mixing it with seed for three weeks, if they show no interest in the veges. After three weeks, put the seeds lower in the cage, and the pellets and veges near their favorite higher perches. This gives them more time to explore their new food, as they tend to trust things higher up in the air. You have to be persistent and offer it daily or they will never transition. It could take months, sometimes years.