I know exactly what you are going through. I also have a tiel who didnt play, and just sat in the cage eating, and squealing. I tried everything I could think of, or was suggested to me. Eventually after 2 years of trying all sorts of everything, even trying her in a cage with budgies (who ganged up on her and pecked at her) I gave in and got her a mate. She changed almost over night. She still wasnt really interested in her toys, but she preened Jim and he preened her and she just seemed so much happier.
Recently my husband and I built an aviary and our tiels are so happy. She still squeals but not continuously and she is answered by Jim so its much cuter to listen to. She now flies around the aviary, and swings and climbs to her little hearts content.
I'm not suggesting you get another tiel to keep her company, but that is the thing that worked for me;)