There are several reasons why this behavior may occur. Check the following properties of the annotation:

1. Make sure the Transparency Slider is far enough to the right.
2. Make sure the layer assigned to the annotation is turned ON.
3. Make sure the font’s color of the annotation is visible.
4. Make sure the fill’s color does not conflict with the font’s color of the annotation.
5. Make sure the font size of the annotation is not too small.