I have the same problem with my cockatiel. We got her from a breeder, she used to eat veggies. By her 3rd month we had her, her diet started going backwards. At first she ate veggies but then less and less. i feed her spinach, carrot, kale and tried to get her to eat apple. She used to love spinach, now she nibbles on it once or twice and she's done, same with everything else. Also taught her to eat pellets, but she only eats them from hand one by one.
I tried so much: pretending to eat, mixing with seeds, presenting foods in different ways, even tried scheduled feeding where I only offered her seeds in mornings and afternoons and in between only greens and pellets. Their untouched by evening and she can even eat her poo time to time if she doesn't find any seeds in cage. Did I mess up? Why ;-; She full on rejects greens. I'm afraid of starving her to death.
She's about half a year so I still have some hope left in me.
Anyone have any more suggestions?