There are two common reasons for this.
First, check to make sure that you didn't accidentally set Typeface Embed to "On" if you're not trying to embed a typeface. If you are trying to embed a typeface, see the Embed Typefaces overview.
The second is if you are using the Flash component, and are applying a mask to the component on your FLA timeline. Flash doesn't allow dynamic text fields using system typefaces to be used underneath masks. The solution here is to either remove the mask, or embed a typeface to use in its text fields (see link above).