Female cockatiels will lay infertile eggs from time to time. I would confirm you have a pair and not two females as if you have two females, you will just keep getting infertile eggs. If you see only a yolk, especially after all this time, then they are certainly not fertile. Fertile bird eggs are usually a pink or red color on the inside. My female is on her 3rd clutch of infertile eggs since we got her a few years ago.