From what I have read, Tiels are notoriously finicky eaters. The books I read said not to take their food out because some tiels will starve before resorting to something new. It went on to state you just have to keep trying in hopes they get curious and try a nibble, at which point if they like it, it becomes easy.
I would try sprinkling seeds or millet on the fruits perhaps to try to attract them to try it. With Dawn, I've also had her out and would nibble on something myself and that would sometimes spark her curiousity to try a nibble too.
Hopefully my information isn't wrong, I know many times different books will tell you different information on this, but this is the info I have gone with
Mom to Tasha, Raven, and Rudy the greyhound
Missing always: Tommy, at the Rainbow Bridge