If your dynamic text field isn't showing up when you test your movie, the font you're using probably isn't properly embedded. Check the font embedding option in the Text Option panel.
Using HTML formatting with dynamic text fields is another reason the text might not show up. Only the fonts and font styles used in the field are automatically embedded in the movie.
When a variable is created by a dynamic text field, the variable type is assumed to be a string and might need to be converted with the Number function before being used in numeric calculations.
Referencing a variable outside a dynamic or an input text field's own timeline in its Variable field can yield inconsistent results.