When you inserted the variable in a text container (caption or shape), did you use the X button in the Properties panel? Do not type in the variable between $$ but use that work flow. In the dialog box you will be able to define the length displayed for the variable. This will be the same for the Scrolling Text Interaction, no need to switch to that because it has the same limit for displaying a variable (used to be 256 characters, but heard it is now less in CP9, didn't double-check).
Moreover, only if you are publishing to SWF will it be possible to show the content in a learning interaction by defining the variable. This is functional for some learning interactions, not for all and it is NOT working for HTML output. That is another reason to use a text container instead of that interaction.
Some examples of using the Scrolling Text Interaction: Custom Short Answer Question - Captivate blog