Hi Angela,
In order to add multiple attributes to the left y-axis, there must be a numeric attribute on the x-axis. Similarly, to add a numeric attribute to the right y-axis, there must be a numeric attribute on both the x- and left-y axes.
I suggest you check carefully to make sure their attributes, especially on the x-axis are being treated as numeric. And, if they are not, use the popup menu obtained by clicking on the attribute name and choose “Treat as Numeric.”
It would be helpful to get an image or a shared file for the situations for which students are having problems.
Bill