Chickens need to learn to eat new foods. Chickens raised by their mother in a flock have an easier time of it, because their mother eats the good things in front of them and that teaches them that particular food is safe and good to eat. Hatchery chickens have to risk death by sampling unknown and potentially poisonous foods on their own. They have no idea when they start living with us, that we are screening their food supplies and only giving them the safe foods. Eventually, I do think they get that idea, after we feed them lots of tasty foods over a period of time. At that point, they usually decide to trust us and at least try whatever we bring them.
Some chickens are more adventurous and they will try things sooner. If you are lucky enough to have at least one in your flock, they will be the flock "guinea pig" and get the others eating a new food sooner. For foods that you don't mind eating, you can eat it in front of your chickens and they will see that it's safe for you to eat. That doesn't mean they will like it, but it does mean they shouldn't snub it as potentially toxic.
I think chickens usually take to hulled sunflower seed right away and take more time to learn to eat the sunflower seeds in the shell. I would try taking some of the seed and giving it a whack with a hammer, to crush the seeds and expose the inside kernel. See if they like the kernels. Once they are eating the kernels, you can try giving them a mix of cracked and uncracked seeds. I'd even do the cracking in front of them, so they understand that the the kernels are hiding inside the shells and it's okay to just eat the seeds whole.
It usually only takes a few minutes to do things in a way that teaches chickens, so I usually do it that way. It ends up being faster for me in the long run.