A canny friend had the solution:
"Remember that the view of your image onscreen is much reduced, so a ‘normal’ font size, like 12-point, is tiny on the image. Also, the default color is silvery-gray, so it does not contrast strongly with many images.
Try selecting a color that will stand out, like red, and increase font size to, say, 72 point, so you can find the text to verify that the tool works."
This worked for me. Thanks to all for your interest.