The AA needs a specific line, depending on the element you are using. They usually require a specific lamp as well.
Is the right lamp either installed or selected by the instrument? For example, is the zinc lamp in the light path when you select zinc?
The wavelenght of the monochromator also has to be selected. Again, using zinc as an example, is the element wavelength selected. If using zinc, the most common wavelength is 213.9 nm. This is actually a doublet, so the correct peak width could also be an issue.
It is a starting point. Start here. Good luck.