I have found that perseverance helps. If I really want to introduce a new food I keep putting it in my bird's cage each day a fresh piece until eventually he tries it. Most recent example was hemp which took him 9 months to try and now he loves it. Of course he is an older bird so with a young one hopefully they will give it a go sooner, but cockatiels tend to be creatures of habit and they are often suspicious of anything new. The other thing to consider is how you are giving it. For example, I used to put broccoli florets in the cage and then I tried cutting up the broccoli in a dish. My bird prefers the cut up broccoli.